3 INFECTIOUS DISEASES

Question 1

(1) Which serum antibody response usually characterizes the primary (early) stage of syphilis?

A. Antibodies against syphilis are undetectable
B. Detected 1 to 3 weeks after appearance of the primary chancre
C. Detected in 50% of cases before the primary chancre disappears
D. Detected within 2 weeks after infection

Answer 1

B. Detected 1 to 3 weeks after appearance of the primary chancre

During the primary stage of syphilis, about 90% of patients develop antibodies between 1 and 3 weeks after the appearance of the primary chancre.

Question 2

(2) What substance is detected in the sample by the rapid plasma reagin (RPR) and Venereal Disease Research Laboratory (VDRL) tests for syphilis?

A. Cardiolipin
B. Anticardiolipin antibody (ACA)
C. Anti–Treponema pallidum antibody
D. T. pallidum

Answer 2

B. Anticardiolipin antibody (ACA)

Reagin is the name for a nontreponemal antibody that appears in the serum of individuals with syphilis and is detected by the RPR and VDRL assays. Reagin reacts with cardiolipin, a lipid-rich extract of beef heart and other animal tissues.

Question 3

(3) What type of antigen is used in the RPR card test?

A. Live treponemal organisms
B. Killed suspension of treponemal organisms
C. Cardiolipin
D. Tanned sheep cells

Answer 3

C. Cardiolipin

Cardiolipin is extracted from animal tissues, such as beef hearts, and attached to carbon particles. In the presence of reagin, the particles will agglutinate.

Question 4

(4) Which of the following is the most sensitive test to detect congenital syphilis?

A. VDRL
B. RPR
C. T. pallidum particle agglutination (TP-PA)
D. Polymerase chain reaction (PCR)

Answer 4

D. Polymerase chain reaction (PCR)

PCR will amplify a very small amount of DNA from T. pallidum and allow for the detection of the organism in the infant. Antibody tests, such as VDRL and RPR, may detect maternal antibody only and do not indicate if the infant has been infected.

Question 5

(5) A biological false-positive reaction is least likely with which test for syphilis?

A. VDRL
B. TP-PA
C. RPR
D. All are equally likely to yield a false-positive result

Answer 5

B. TP-PA

The TP-PA test is more specific for T. pallidum compared with nontreponemal tests, such as the VDRL and RPR tests, and would be the least likely to yield a biological false-positive result. Nontreponemal tests have a biological false-positive rate of 1% to 10%, depending on the patient population tested. False-positive findings are caused commonly by infectious mononucleosis (IM), SLE, viral hepatitis, and human immunodeficiency virus (HIV) infection.

Question 6

(6) A 12-year old girl has symptoms of fatigue and localized lymphadenopathy. Laboratory tests reveal peripheral blood lymphocytosis, positive RPR, and positive spot test for IM. What test should be performed next?

A. HIV screen
B. VDRL
C. Epstein-Barr virus (EBV)–specific antigen test
D. TP-PA test

Answer 6

D. TP-PA test

The patient’s symptoms are nonspecific and could be attributed to many potential causes. However, the patient’s age, lymphocytosis, and serological results point to IM. The rapid spot test for antibodies seen in IM is highly specific. The EBV specific antigen test is more sensitive but is unnecessary when the spot test is positive. HIV infection is uncommon at this age and is often associated with generalized lymphadenopathy and a normal or reduced total lymphocyte count. IM antibodies are commonly implicated as a cause of biological false-positive nontreponemal test results for syphilis. Therefore, a treponemal test for syphilis should be performed to document this phenomenon in this case.

Question 7

(7) Which test is most likely to be positive in the tertiary stage of syphilis?

A. Treponemal-specific antibody
B. RPR
C. VDRL
D. Reagin screen test (RST)

Answer 7

A. Treponemal-specific antibody

A treponemal-specific antibody test is more likely to be positive compared with a nontreponemal test in the tertiary stage of syphilis. In some cases, systemic lesions have subsided by the tertiary stage, and the nontreponemal tests become seronegative. Although the treponemal-specific antibody test is the most sensitive test for tertiary syphilis, it will be positive in both treated and untreated cases.

Question 8

(8) What is the most likely interpretation of the following syphilis serological results?
RPR: reactive; TP-PA: nonreactive

A. Neurosyphilis
B. Secondary syphilis
C. Syphilis that has been successfully treated
D. Biological false positive

Answer 8

D. Biological false positive

A positive reaction with nontreponemal antigen and a negative reaction with a treponemal antigen is most likely caused by a biological false-positive nontreponemal test result.

Question 9

(9) Which specimen is the sample of choice to evaluate latent or tertiary syphilis?

A. Serum sample
B. Chancre fluid
C. Cerebrospinal fluid (CSF)
D. Joint fluid

Answer 9

C. Cerebrospinal fluid (CSF)

Latent syphilis usually begins after the second year of untreated infection. In some cases, the serological tests become negative. However, if neurosyphilis is present, CSF serology will be positive and the CSF will display increased protein and pleocytosis characteristic of central nervous system infection.

Question 10

(10) Interpret the following quantitative RPR test results.
RPR titer: weakly reactive—1:4; reactive—1:8 to 1:64

A. Excess antibody, prozone effect
B. Excess antigen, postzone effect
C. Equivalence of antigen and antibody
D. Impossible to interpret; testing error

Answer 10

A. Excess antibody, prozone effect

This patient may be in the secondary stage of syphilis and is producing large amounts of antibody to T. pallidum sufficient to cause a prozone reaction as a result of antibody excess in the test. The test became strongly reactive only after the antibody was diluted.

Question 11

(11) Tests to identify infection with HIV fall into which three general classification types of tests?

A. Tissue culture, antigen, and antibody tests
B. Tests for antigens, antibodies, and nucleic acid
C. DNA probe, DNA amplification, and Western blot tests
D. ELISA, Western blot, and Southern blot tests

Answer 11

B. Tests for antigens, antibodies, and nucleic acid

The fourth- and fifth-generation HIV assays detect both antibodies to HIV and the HIV p24 antigen. Molecular assays can be used to resolve discrepant screening results or to confirm results, as well as to quantitate the amount of virus present.

Question 12

(12) Which tests are considered screening tests for HIV?

A. ELISA, chemiluminescent, and rapid antibody tests
B. IFA, Western blot, radioimmunoprecipitation assay
C. Culture, antigen capture assay, DNA amplification
D. Reverse transcriptase and messenger RNA (mRNA) assay

Answer 12

A. ELISA, chemiluminescent, and rapid antibody tests

The fourth- and fifth-generation HIV assays detect both antibody and the p24 antigen. These assays are available in ELISA, automated chemiluminescent systems, and mutilplex systems and in a rapid card format.

Question 13

(13) Which tests are the recommended confirmatory tests for HIV?

A. ELISA and rapid antibody tests
B. HIV-1,2 antibody differentiation assays, and qualitative PCR test
C. Culture, antigen capture assay, quantitative PCR
D. Reverse transcriptase and mRNA assay

Answer 13

B. HIV-1,2 antibody differentiation assays, and qualitative PCR test

The current HIV testing algorithm begins with a screening assay, followed by an HIV-1,2 antibody differentiation assay (HIV-1,2 supplemental assay) and, if those results are discordant, an HIV qualitative PCR assay is performed. Western blot is not included in this algorithm.

Question 14

(14) How do fourth- and fifth-generation HIV tests reduce the time from infection to the test becoming positive?

A. They are PCR tests detecting viral RNA
B. They detect p24 antigen in addition to HIV antibody
C. They detect proviral DNA
D. They detect antibodies to more antigens than earlier generations of HIV tests

Answer 14

B. They detect p24 antigen in addition to HIV antibody

Including the p24 antigen in the fourth- and fifth generation tests allows for the detection of HIV infection approximately 1 week earlier compared with the third-generation antibody-only assays and up to 3 weeks earlier compared with Western blot.

Question 15

(15) A woman who has had five pregnancies subsequently tests positive for HIV on a fourth-generation assay and is negative on an HIV-1,2 differentiation assay and a follow-up molecular assay. The initial reactivity may be caused by:

A. Possible cross-reaction with herpes or EBV
antibodies
B. Interference from medication
C. Cross-reacting antibodies elicited during pregnancy
D. Possible technical error; a repeat specimen should be requested

Answer 15

C. Cross-reacting antibodies elicited during pregnancy

Pregnancy is a common cause of false-positive HIV screening results.

Question 16

(16) Interpret the following results for HIV testing: Fourth-generation ELISA: positive; repeat ELISA: positive; HIV 1,2 antibody differentiation assay: negative; qualitative HIV RNA rtPCR assay: positive

A. False-positive fourth-generation assay
B. False-negative antibody differentiation assay
C. Indeterminate; further testing indicated
D. HIV p24 antigen detected on fourth-generation ELISA

Answer 16

D. HIV p24 antigen detected on fourth-generation ELISA

The fourth-generation HIV assay detects antibody and the p24 antigen but does not differentiate between those results. In this case, the antibody confirming test is negative, suggesting the initial reactive fourth-generation test result is either a false-positive one or is caused by the presence of p24 in the specimen. The positive molecular assay confirms the presence of the virus in the specimen. This usually occurs in early infection, prior to antibody being produced.

Question 17

(17) What is the most likely explanation when antibody tests for HIV are negative but the PCR test is positive?

A. Probably not HIV infection
B. Patient is in the “window phase” before antibody production
C. Tests were performed incorrectly
D. Clinical signs may be misinterpreted

Answer 17

B. Patient is in the “window phase” before antibody production

In early seroconversion, patients may not be making antibodies in sufficient amounts to be detected by antibody tests. The period between infection with HIV and the appearance of detectable antibodies is called the window phase. This period has been reduced to a few weeks by antibody and antigen detecting fourth- and fifth-generation assays, and an algorithm that includes PCR testing.

Question 18

(18) What criteria constitute the classification system for HIV infection?

A. CD4-positive T-cell count and clinical symptoms
B. Clinical symptoms, condition, duration, and strength of reactivity on a fourth-generation HIV test
C. Presence or absence of lymphadenopathy
D. Strong fourth-generation HIV test reactivity and CD8-positive T-cell count

Answer 18

A. CD4-positive T-cell count and clinical symptoms

The classification (not diagnostic) system for HIV infection is based on a combination of CD4-positive T-cell count (helper T cells) and various categories of clinical symptoms. Classification is important in determining treatment options and the progression of the disease.

Question 19

(19) What is the main difficulty associated with the development of an HIV vaccine?

A. The virus has been difficult to culture; antigen extraction and concentration are extremely laborious
B. Human trials cannot be performed
C. Different strains of the virus are genetically diverse
D. Anti-idiotype antibodies cannot be developed

Answer 19

C. Different strains of the virus are genetically diverse

Vaccine development has been difficult primarily because of the genetic diversity among different strains of the virus, and new strains are constantly emerging. HIV-1 can be divided into two main subtypes designated M (for main) and O (for outlier). The M group is further divided into nine subgroups, designated A through J (there is no E subgroup), based on differences in the nucleotide sequence of the gag gene. Two remaining subtypes are designated N (non-M and non-O) and P (a subtype related to SIVgor). A vaccine that is effective for all of the subgroups of HIV-1 has yet to be developed.

Question 20

(20) Which CD4:CD8 ratio is most likely in a patient with AIDS?

A. 2:1
B. 3:1
C. 2:3
D. 1:3

Answer 20

D. 1:3

An inverted CD4:CD8 ratio (less than 1.0) is a common finding in a patient with AIDS. The Centers for Disease Control and Prevention (CDC) requires a CD4-positive (helper T) cell count of less than 200 µL or 14% in the absence of an AIDS-defining illness (e.g., Pneumocystis carinii pneumonia) in the case surveillance definition of AIDS.

Question 21

(21) What is the advantage of fourth-generation rapid HIV tests over earlier rapid HIV tests?

A. They use recombinant antigens
B. They detect multiple strains of HIV
C. They detect p24 antigen
D. They are quantitative

Answer 21

C. They detect p24 antigen

Both third-generation and fourth-generation rapid tests for HIV use recombinant and synthetic HIV antigens conjugated to a solid phase. The multivalent nature of these tests allows for detection of less common subgroups of HIV-1 and simultaneous detection of both HIV-1 and HIV-2. However, the fourth-generation assays also use solid-phase antibodies to p24 antigen to detect its presence. Because p24 antigen appears before antibodies to HIV, fourth-generation tests can detect infection 4 to 7 days earlier compared with tests based on antibody detection alone.

Question 22

(22) Which method is used to test for HIV infection in infants who are born to HIV-positive mothers?

A. ELISA
B. Western blot test
C. PCR test
D. Viral culture

Answer 22

C. PCR test

Fourth- and fifth-generation ELISA and chemiluminescent assays reflect the presence of maternal antibody. The PCR test uses small amounts of blood and does not rely on the antibody response. PCR amplifies small amounts of viral nucleic acid and can detect less than 20 copies of viral RNA per milliliter of plasma. These qualities make PCR ideal for the testing of infants. Nucleic acid methods for HIV RNA include both qualitative (for diagnosis) and quantitative (for monitoring) reverse-transcriptase real-time PCR (RT-PCR) assays.

Question 23

(23) What is the most likely cause when a fourth generation HIV assay is positive for all controls and samples?

A. Improper pipetting
B. Improper washing
C. Improper addition of sample
D. Improper reading

Answer 23

B. Improper washing

Improper washing may not remove unbound, enzyme-conjugated antihuman Ig, and every sample may appear positive.

Question 24

(24) What constitutes a diagnosis of viral hepatitis?

A. Abnormal test results for liver enzymes
B. Clinical signs and symptoms
C. Positive results for hepatitis markers
D. All of these options

Answer 24

D. All of these options

To diagnose a case of hepatitis, the physician must consider clinical signs as well as the results of laboratory tests that measure liver enzymes and hepatitis markers.

Question 25

(25) Which of the following statements regarding infection with hepatitis D virus (HDV) is true?

A. Occurs in patients with HIV infection
B. Does not progress to chronic hepatitis
C. Occurs in patients with hepatitis B virus (HBV) infection
D. Is not spread through blood or sexual contact

Answer 25

C. Occurs in patients with hepatitis B virus (HBV) infection

HDV is an RNA virus that requires the surface antigen or envelope of the HBV for entry into the hepatocyte. Consequently, HDV can infect only patients who are coinfected with hepatitis B.

Question 26

(26) All of the following hepatitis viruses are spread through blood or blood products except:

A. Hepatitis A virus (HAV)
B. HBV
C. HCV
D. HDV

Answer 26

A. Hepatitis A virus (HAV)

HAV is spread through the fecal–oral route and is the cause of infectious hepatitis. HAV has a shorter incubation period (2–7 weeks) than HBV (1–6 months). Epidemics of HAV can occur, especially when food and water become contaminated with raw sewage. Hepatitis E virus is also spread via the oral–fecal route and, like HAV, has a short incubation period.

Question 27

(27) Which hepatitis B marker is the best indicator of early acute infection?

A. Hepatitis B surface antigen (HBsAg)
B. Hepatitis B e-antigen (HBeAg)
C. Hepatitis B core antibody (anti-HBc)
D. Hepatitis B surface antibody (anti-HBs)

Answer 27

A. Hepatitis B surface antigen (HBsAg)

HBsAg is the first marker to appear in HBV infection. It is usually detected within 4 weeks of exposure (prior to the rise in transaminases) and persists for about 3 months after serum enzyme levels return to normal.

Question 28

(28) Which is the first antibody detected in serum after infection with HBV?

A. Anti-HBs
B. Anti-HBc IgM
C. Anti-HBe
D. All are detectable at the same time

Answer 28

B. Anti-HBc IgM

Antibody to the hepatitis B core antigen (anti-HBc) is the first detectable hepatitis B antibody. It persists in serum for years after infection and is found in the serum of asymptomatic carriers of HBV. Because levels of total anti-HBc are high after recovery, IgM anti-HBc is a more useful marker for acute infection. Both anti-HBc and anti-HBs can persist for life, but only anti-HBs is considered protective.

Question 29

(29) Which antibody persists in low-level carriers of HBV?

A. IgM anti-HBc
B. IgG anti-HBc
C. IgM anti-HBe
D. IgG anti-HBs

Answer 29

B. IgG anti-HBc

IgG anti-HBc can be detected in carriers who are HBsAg and anti-HBs negative. These persons are hepatitis B DNA positive also and, thus, are presumed infective, even though the level of HBsAg is too low to detect. No specific B core IgG test is available, however. This patient would be positive in the anti-B core total antibody assay and negative in the anti-HB core IgM test.

Question 30

(30) What is the most likely explanation when a patient has clinical signs of viral hepatitis but tests negative for HAV IgM, HBsAg, and HCV antibody?

A. Tests were performed improperly
B. The patient does not have hepatitis
C. The patient may be in the “core window”
D. Clinical evaluation was performed improperly

Answer 30

C. The patient may be in the “core window”

The patient may be in the “core window,” the period of HBV infection when both the surface antigen and surface antibody are undetectable. The IgM anti HBc and the anti-HBc total antibody assays, along with the hepatitis B DNA PCR assay would be the only detectable markers in the serum of a patient in the core window phase of HBV infection.

Question 31

(31) Which hepatitis B markers should be performed on blood products?

A. HBsAg and anti-HBc
B. Anti-HBs and anti-HBc
C. HBeAg and HBcAg
D. Anti-HBs and HBeAg

Answer 31

A. HBsAg and anti-HBc

Blood products are tested for HBsAg, an early indicator of infection, and anti-HBc, a marker that may persist for life. Following recovery from HBV infection, some patients demonstrate negative serology for HBsAg and anti-HBs but are positive for anti-HBc. Such patients are considered infective.

Question 32

(32) Which hepatitis antibody confers immunity against reinfection with HBV?

A. Anti-HBc IgM
B. Anti-HBc IgG
C. Anti-HBe
D. Anti-HBs

Answer 32

D. Anti-HBs

Anti-HBs appears later in infection compared with anti-HBc and is used as a marker for immunity after infection or vaccination, rather than for diagnosis of current infection.

Question 33

(33) Which test, other than serological markers, is most consistently elevated in viral hepatitis?

A. Antinuclear antibodies
B. Alanine aminotransferase (ALT)
C. Absolute lymphocyte count
D. Lactate dehydrogenase

Answer 33

B. Alanine aminotransferase (ALT)

ALT is a liver enzyme and may be increased in hepatic disease. Highest levels occur in acute viral hepatitis, reaching 20 to 50 times the upper limit of normal.

Question 34

(34) If only anti-HBs is positive, which of the following can be ruled out?

A. HBV vaccination
B. Distant past infection with HBV
C. Hepatitis B immune globulin (HBIG) injection
D. Chronic HBV infection

Answer 34

D. Chronic HBV infection

Persons with chronic HBV infection show a positive test result for anti-HBc (IgG or total) and HBsAg but not anti-HBs. Patients with active chronic hepatitis have not become immune to the virus.

Question 35

(35) Interpret the following results for EBV infection: IgG and IgM antibodies to viral capsid antigen (VCA) are positive.

A. Infection in the past
B. Infection with a mutual enhancer virus, such as HIV
C. Current infection
D. Impossible to interpret; need more information

Answer 35

C. Current infection

IgM and IgG antibodies to VCA are found in a current infection with EBV. The IgG antibody may persist for life, but the IgM anti-VCA disappears within 4 months after the infection resolves.

Question 36

(36) Rapid mono tests use latex particles coated with which of the following?

A. Guinea pig antigen
B. Beef proteins
C. Horse proteins
D. Sheep proteins

Answer 36

B. Beef proteins

Rapid mono tests detect a heterophile antibody directed against beef proteins. Although these antibodies may also react with horse or sheep red blood cells (RBCs), those proteins are not used in these tests.

Question 37

(37) Blood products are tested for which virus
before being transfused to newborns?

A. EBV
B. Human T-lymphotropic virus II (HTLV-II)
C. CMV
D. HDV

Answer 37

C. CMV

CMV can be life threatening if transmitted to a newborn through a blood product. HTLV-II is a rare virus, which like HIV, is a T-cell tropic RNA retrovirus. The virus has been associated with hairy cell leukemia, but this is not a consistent finding.

Question 38

(38) What is the endpoint for the antistreptolysin O (ASO) latex agglutination assay?

A. Highest serum dilution that shows no agglutination
B. Highest serum dilution that shows agglutination
C. Lowest serum dilution that shows agglutination
D. Lowest serum dilution that shows no agglutination

Answer 38

B. Highest serum dilution that shows agglutination

The latex test for ASO includes latex particles coated with streptolysin O. Serial dilutions are prepared and the highest dilution showing agglutination is the endpoint.

Question 39

(39) A streptozyme test was performed, but the result was negative, even though the patient showed clinical signs of a streptococcal throat infection. What should be done next?

A. Either ASO or anti-deoxyribonuclease B (anti DNase B) test
B. Another streptozyme test using diluted serum
C. Antihyaluronidase test
D. Wait for 3 to 5 days and repeat the streptozyme test

Answer 39

A. Either ASO or anti-deoxyribonuclease B (anti DNase B) test

The streptozyme test is used for screening and contains several of the antigens associated with streptococcal products. Because some patients produce an antibody response to a limited number of streptococcal products, no single test is sufficiently sensitive to rule out infection. Clinical sensitivity is increased by performing additional tests when initial results are negative. The streptozyme test generally yields more false-positive and false-negative results compared with the ASO and anti-DNase B tests. A positive result occurs in a smaller number of patients with recent streptococcal infections in the antihyaluronidase test compared with the ASO and anti-DNase B tests.

Question 40

(40) Rapid assays for influenza that utilize specimens obtained from nasopharyngeal swabs
detect:

A. IgM anti-influenza
B. IgA anti-influenza
C. IgA–influenza antigen immune complexes
D. Influenza nucleoprotein antigens

Answer 40

D. Influenza nucleoprotein antigens

The rapid influenza assays are antigen detection methods. They are designed to detect early infection, before antibody is produced.

Question 41

(41) How can interfering cold agglutinins be removed from a test sample?

A. Centrifuge the serum and remove the top layer
B. Incubate the clot at 1°C to 4°C for several hours and then remove the serum
C. Incubate the serum at 56°C in a water bath for 30 minutes
D. Use an anticoagulated sample

Answer 41

B. Incubate the clot at 1°C to 4°C for several hours and then remove the serum

Cold agglutinins will attach to autologous RBCs if incubated at 1°C to 4°C. The absorbed serum will be free of cold agglutinins.

Question 42

(42) All tubes (dilutions) except the negative control are positive for cold agglutinins. This indicates:

A. Contaminated RBCs
B. A rare antibody against RBC antigens
C. The sample was stored at 4°C prior to separating serum and cells
D. Further serial dilution is necessary

Answer 42

D. Further serial dilution is necessary

Cold agglutinins may be measured in patients who have cold agglutinin disease, that is, cold autoimmune hemolytic anemia. In such cases, titers can be as high as 106. If all tubes (dilutions) for cold agglutinins are positive, except the negative control, then a high titer of cold agglutinins is present in the sample. Further serial dilutions should be performed.

Question 43

(43) All positive cold agglutinin tubes remain positive after 37°C incubation except the positive control. What is the most likely explanation for this situation?

A. High-titer cold agglutinins
B. Contamination of the test system
C. Antibody other than cold agglutinins
D. Faulty water bath

Answer 43

C. Antibody other than cold agglutinins

Cold agglutinins do not remain reactive above 30°C, and agglutination must disperse after incubation at 37°C. The most likely explanation when agglutination remains after 37°C incubation is that a warm alloantibody or autoantibody is present.

Question 44

(44) Which increase in antibody titer (dilution) best indicates an acute infection?

A. From 1:2 to 1:8
B. From 1:4 to 1:16
C. From 1:16 to 1:256
D. From 1:64 to 1:128

Answer 44

C. From 1:16 to 1:256

A fourfold (two-tube) or greater increase in antibody titer is usually indicative of an acute infection. Although answers A and B show a fourfold rise in titer, answer C shows a 16-fold rise in titer and is the most definitive. In most serological tests, a single high titer is insufficient evidence of acute infection unless specific IgM antibodies are measured because age, individual variation, immunologic status, and history of previous exposure (or vaccination) cause a wide variation in normal serum antibody titers.

Question 45

(45) Which of the following positive antibody tests may be an indication of recent vaccination or early primary infection for rubella in a patient with no clinical symptoms?

A. Only IgG antibodies positive
B. Only IgM antibodies positive
C. Both IgG and IgM antibodies positive
D. Fourfold rise in titer for IgG antibodies

Answer 45

B. Only IgM antibodies positive

If only IgM antibodies are positive, this result indicates recent vaccination or early primary infection.

Question 46

(46) Why is laboratory diagnosis difficult in cases of Lyme disease?

A. Clinical response may not be apparent upon initial infection; IgM antibody may not be detected until 3 to 6 weeks after the infection
B. Laboratory tests may be designed to detect whole Borrelia burgdorferi, not flagellar antigen found early in infection
C. Most laboratory tests are technically demanding and lack specificity
D. Antibodies formed initially to B. burgdorferi may cross react in antigen tests for autoimmune diseases

Answer 46

A. Clinical response may not be apparent upon initial infection; IgM antibody may not be detected until 3 to 6 weeks after the infection

Lyme disease is caused by B. burgdorferi, a spirochete, and typical clinical symptoms, such as rash or erythema chronicum migrans, may be absent in some infected individuals. Additionally, IgM antibody is not detectable by laboratory tests until 3 to 6 weeks after a tick bite, and IgG antibody develops later.

Question 47

(47) Serological tests for which disease may give a false-positive result if the patient has Lyme disease?

A. HIV
B. Syphilis
C. EBV
D. Hepatitis C

Answer 47

B. Syphilis

Lyme disease is caused by a spirochete, and positive results may occur with some specific treponemal antibody tests for syphilis.

Question 48

(48) In monitoring a patient with HIV infection, which parameter may be expected to be the most sensitive indicator of the effectiveness of antiretroviral treatment?

A. HIV antibody titer
B. CD4:CD8 ratio
C. HIV viral load
D. Absolute total T-cell count

Answer 48

C. HIV viral load

The HIV viral load will rise or fall in response to treatment more quickly compared with any of the other listed parameters. The absolute CD4 count is also an indicator of treatment effectiveness and is used in resource-poor areas that might not have facilities for molecular testing. Note, however, that the absolute CD4 count is not one of the choices.

Question 49

(49) A renal transplant recipient is found to have a rising creatinine level and reduced urine output. The physician orders a “urine PCR” assay. When you call to find out what organism the physician wants to identify, you are told:

A. HCV
B. Legionella pneumophila
C. EBV
D. BK virus

Answer 49

D. BK virus

BK virus is a polyoma virus that can cause renal and urinary tract infections. The virus is an opportunistic pathogen and has become a well-recognized cause of poor renal function in kidney transplant recipients. Antibody testing is not practical or useful for this infection. The principal diagnostic assays are urinary cytology, and specific BK virus PCR testing in urine and serum. Although L. pneumophila can be diagnosed through a urinary antigen assay, that organism is not a primary cause of renal insufficiency in transplant recipients.

Question 50

(50) A newborn is to be tested for vertically transmitted HIV infection. Which of the following tests is most useful?

A. HIV PCR
B. CD4 count
C. Rapid HIV antibody test
D. HIV IgM antibody test

Answer 50

A. HIV PCR

Neonatal HIV diagnosis is performed by screening for the presence of the virus. The current antibody tests are either an IgG-specific assay or an IgG/IgM combination assay. Thus, an infant whose mother is HIV positive will also be positive in the HIV antibody assay. Although the CD4 count may be a useful assay to determine disease activity, there are many causes of reduced CD4 numbers, so this assay should not be used to diagnose HIV infection.

Question 51

(51) Which of the following fungal organisms is best diagnosed by an antigen detection test as opposed to an antibody detection assay?

A. Histoplasma
B. Cryptococcus
C. Candida
D. Aspergillus

Answer 51

B. Cryptococcus

The Cryptococcus antibody response is not a reliable indicator of a current infection; thus, an antigen assay is normally used to monitor the disease. The antigen assay may be used for serum or CSF and will decline in response to treatment much faster than a traditional antibody test. A urinary antigen test is available for histoplasmosis, and a serum galactomannan assay is available for Aspergillus. Those two assays perform better than antibody detection. No antigen test is available for Candida, and thus, antibody detection is the best serological procedure for this organism.

Question 52

(52) Your cytology laboratory refers a Papanicolaou smear specimen to you for an assay designed to detect the presence of a virus associated with cervical cancer. You perform:

A. An ELISA for anti-human simplex virus 2 (anti HSV-2) antibodies
B. A molecular assay for HSV-2
C. An ELISA for human papilloma virus (HPV) antibodies
D. A molecular assay for HPV

Answer 52

D. A molecular assay for HPV

Cervical cell atypia and cervical cancer are associated with specific high-risk serotypes of HPV infections. Although HPV antibody assays are available, they are not serotype specific, nor do they relate to disease activity. Thus, molecular probe assays are the tests of choice to detect high-risk HPV infection. Although HSV-2 is associated with genital herpesvirus, that virus has not been shown to cause cervical cancer.

Question 53

(53) An immunosuppressed patient has unexplained anemia. The physician suspects a parvovirus B19 infection. The parvovirus IgM test result is negative. The next course of action is to tell the physician that:

A. The patient does not have parvovirus
B. A convalescent specimen is recommended in 4 weeks to determine if a fourfold rise in titer has occurred
C. A parvovirus PCR is recommended
D. A recent transfusion for the patient’s anemia may have resulted in a false-negative result and the patient should be retested in 4 weeks

Answer 53

C. A parvovirus PCR is recommended

A negative IgM assay rarely rules out an infection. Although a convalescent specimen may be useful in many cases, in an immunosuppressed patient, the convalescent specimen may remain negative in the presence of an infection. Thus, a parvovirus PCR test is the preferred choice in this case. A false negative result could be caused by multiple whole blood or plasma transfusions, but retesting for antibody a month later would not be beneficial to the patient.