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Infectious Disease > Lab Medicine > Flashcards

Lab Medicine Flashcards

1
Q

When would you want to order a blood culture?

A

When you are seeking out systemic infection

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2
Q

What is the gold standard for diagnosing strep throat?

A

A throat culture

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3
Q

What is the specific organism responsible for strep throat?

A

Streptococcus pyogenes

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4
Q

How long does it take for throat culture results to come back?

A

24-48 hours

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5
Q

Other than for strep throat, when do you want to order a throat culture?

A

When diphtheria, gonococcal pharyngitis, or thrush (Candida) is suspected

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6
Q

Why would you not do a gram stain on a throat swab?

A

Because it is impossible to visually distinguish between normal flora streptococci and strep pyogenes

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7
Q

When would you do a sputum culture?

A

When pneumonia or TB is suspected

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8
Q

If you suspect pneumonia or TB, but the patient can’t cough, how do you get a sputum culture?

A
  • Induction of sputum
  • Transtracheal aspirate
  • Bronchial lavage
  • Lung biopsy
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9
Q

What is the most common cause of community acquired pneumonia?

A

S. pneumoniae

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10
Q

What organism is the sputum gram stain not sensitive or specific for?

A

S. pneumoniae

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11
Q

When is the sputum gram stain useful?

A

In broadening initial coverage in patients to be hospitalized for CAP

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12
Q

When do you order a bacterial wound culture?

A
  • To determine whether a wound is infected
  • To identify the bacteria causing the infection
  • To prepare for susceptibility testing when required
  • To determine whether wound treatment was effective
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13
Q

Are gram stains performed on a wound culture?

A

Yes; but be aware that no bacteria on stain does not rule out a wound infection

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14
Q

What are the pathogens that frequently cause abscesses of the brain, lungs, and abdomen?

A
  • Anaerobes (bacteroides fragilis)

- Gram positive cocci (staph aureus, strep pyogenes)

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15
Q

What are the pathogens that frequently cause traumatic open wound infections?

A

Soil flora (eg. clostridium perfringens)

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16
Q

What are the pathogens that frequently cause surgical wound infections?

A

Skin flora (eg. staph aureus)

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17
Q

What are the pathogens that frequently cause infections from bites?

A

Dog or cat bites: pasteurella multocida

Human bites: mouth anaerobes

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18
Q

Are aerobes or anaerobes more frequently involved in wounds?

A

Anaerobes; so when culturing, make sure to place specimens in proper collection tubes and promptly transport them to the lab!

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19
Q

Since wound infections are frequently due to multiple organisms, what should you do when culturing?

A

Culture specimen on different media under different atmospheric conditions

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20
Q

When should you perform a CSF culture?

A

When meningitis is suspected

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21
Q

What are the most important causes of acute bacterial meningitis?

A

Three encapsulated organisms

  • Neisseria meningitidis (meningococcal)
  • Strep pneumoniae (pneumococcal aka streptococcal)
  • H. influenzae
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22
Q

In what cases might you get negative CSF cultures?

A

Encephalitis, brain abscess, subdural empyema

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23
Q

What guides the empirical treatment of acute bacterial meningitis?

A

Gram-stained smear of the sample

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24
Q

What is the most rapid way to detect the pathogenic organism involve in acute bacterial meningitis?

A

Immunofluorescence

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25
Q

What are the most common organisms involved in subacute meningitis?

A
  • Mycobacterium tuberculosis

- Cryptococcus neoformans (a fungi)

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26
Q

How do you detect cryptococcus neoformans in spinal fluid?

A

Use India ink!

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27
Q

What are special steps that must be taken in the case of suspicion of tuberculosis meningitis?

A
  • Acid fast stains should be performed’
  • Fluid should be cultured and held for a minimum of 6 weeks
  • M.tuberculosis can be present in small numbers
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28
Q

When should a urine culture be obtained?

A

-When pyelonephritis or cystitis are suspected

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29
Q

What is the most frequent cause of uncomplicated UTI?

A

Escherichia coli

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30
Q

Other than E.coli, what are other common agents of UTI?

A

Enterobacter, Proteus, Enterococcus faecalis

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31
Q

Since urine acquires organisms of natural flora as it passes through the distal portion of the urethra, what are some tips to avoid obtaining these organisms in your urine culture?

A
  • Clean midstream catch
  • Catheterized sample
  • Suprapubic aspirate
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32
Q

What must be found to diagnose significant bacteriuria in asymptomatic persons?

A

Bacterial count of 100,000/mL from a urine sample

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33
Q

What must be found to diagnose significant bacteriuria in symptomatic persons?

A

Bacterial count of 100/mL from a urine sample

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34
Q

What kind of tests look at the efficacy of antibiotics against the growth of an organism?

A

Sensitivity tests

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35
Q

How do you diagnose viral diseases using clinical specimens?

A

5 approaches:

  • Cell culture
  • Direct microscopic identification
  • Rise in antibody titer or presence of IgM antibody
  • Viral antigens in blood or body fluids
  • Viral nucleic acids in blood or the patient’s cells
36
Q

The growth of viruses requires what kind of cultures?

A

Cell cultures! Because viruses only replicate in living cells, not on cell-free media

37
Q

What is CPE?

A

A change in the appearance of the virus-infected cell

Virus growth in a cell culture frequently produces a CPE that can provide a presumptive ID

38
Q

How no CPE is elicited on a cell culture, how is viral presence detected?

A

Direct microscopy (among other things)

39
Q

What are the two types of fungi?

A

Yeasts and molds

40
Q

How do yeasts grow?

A

As single cells that reproduce by asexual budding

41
Q

How do molds grow?

A

As long filaments (hyphae) that form a mat (mycelium)

  • Some hyphae form transverse walls (septate)
  • Other hyphae do not form walls (nonseptate)
42
Q

What quality do many medically important fungi share?

A

They are thermally dimorphic! Aka, they form different structures at different temperatures

  • They exist as molds in the environment at ambient temperature
  • They exist as yeasts in human tissues at body temperature
43
Q

How do you identify fungal diseases?

A

Four approaches

  • Culture of the organism
  • Direct microscopic examination
  • DNA probe tests
  • Serologic tests
44
Q

What kind of agar are fungi frequently cultured on?

A

Sabouraud’s agar

45
Q

What is so special about Sabouraud’s agar?

A
  • Facilitates appearance of slow-growing fungi for inhibiting bacterial growth
  • Supports fungal growth for several weeks
46
Q

For what types of specimens is direct microscopic examination ideal for fungal identification?

A

-Specimens such as sputum, lung biopsy material, and skin scrapings (depend on finding characteristic asexual spores, hyphae, or yeasts in the light microscope)

47
Q

What distinguishing feature do you see during direct microscopic examination of cryptococcus neoformans from CSF with India ink preparations?

A

The wide capsule

48
Q

What is the benefit of using DNA probes in fungal identificaiton?

A

-Can identify colonies growing in culture earlier than visual detection, and diagnosis can be made more rapidly

49
Q

What is the goal of serologic tests in fungal identification?

A
  • Tests for the presence of antibodies in the patient’s serum or CSF
  • Useful in diagnosing systemic mycoses
50
Q

What are two bacteria that are acid fast?

A

Mycobacteria

Norcardia asteroides

51
Q

What are mycobacteria?

A

Aerobic, acid-fast bacilli (rods)

52
Q

What are the major mycobacteria pathogens?

A
  • Mycobacterium tuberculosis

- Mycobacterium leprae

53
Q

What are atypical mycobacgteria?

A
  • Mycobacterium avium-intracellulare complex
  • Mycobacterium kansasii

Can cause tuberculosis-like disease; less frequently pathogens in immunocompromised

54
Q

Instead of gram stained sputum, what are more often used for specific diagnosis of mycoplasma?

A

Serologic testes

55
Q

What is the Cold Agglutinin Test and what is it used for?

A
  • A mycoplasma serology test
  • Patients with certain illnesses develop autoimmune antibodies that agglutinate human red blood cells in the cold (4C) but not at 37C
56
Q

In what diseases have cold agglutinin antibodies been observed?

A
  • M.pneumoniae
  • Adenovirus
  • Infectious tuberculosis
57
Q

When is a stool culture performed?

A

Primarily for cases of enterocolitis

58
Q

What are the most common bacteria causing diarrhea in the US?

A
  • Shigella
  • Salmonella
  • Campylobacter
  • (not as common, but E.coli strains also important)
59
Q

What does a methylene blue stain that reveals many leukocytes in a stool culture indicate?

A

An invasive organism rather than a toxigenic one

60
Q

Is a gram stain routinely done from a stool culture?

A

No; large numbers of bacteria in the normal colon flora make the interpretation difficult

61
Q

How is intestinal amebiasis diagnosed?

A

By finding either trophozoites in diarrheal stools or cysts in formed stools

62
Q

How many stools for ova/parasites should be examined when cysts are suspected?

A

At least 3, since cysts are passed intermittently

63
Q

What does a complete examination for cysts include and what is the purpose?

A
  • Wet mount in saline
  • An iodine-stained wet mount
  • A fixed, trichrome-stained preparation

Each of these steps brings out different aspects of cyst morphology; helps distinguish amebic from bacillary dysentery

64
Q

What is seen in bacilliary but not in amebic dysentary?

A

Inflammatory cells such as PMNs seen in bacilliary, but not in amebic dysentary

65
Q

How is giardia diagnosed?

A

Direct microscopy

ELISA test that detects a giardia cyst wall antigen in the stool is also useful

66
Q

When would you do a string test, aka an Enterotest?

A

When symptoms indicating giardia persist but the direct microscopy and ELISA tests are negative

67
Q

How do you perform a String test (aka the Enterotest?

A
  • Swallow a gelatin capsule attached to a long string
  • Tape string to your cheek and leave in place 4-6 hours or overnight
  • Withdraw the string and examine under the microscope
  • Trophozoites adhere to string and can be visualized
68
Q

How do you identify malaria?

A

Thick and thin blood smear; diagnosis is made by visualizing the characteristic organisms

69
Q

What is the most frequently used nucleic acid amplification test (NAAT)?

A

PCR! Aka polymerase chain reaction

70
Q

What is PCR used for?

A

To detect microbiologic organisms and genetic diseases

71
Q

What does PCR do?

A

It amplifies the normal DNA replication process; genetic material is identified by agarose gel electrophoresis

72
Q

What is a potential drawback of PCR?

A

Contamination with extraneous DNA

73
Q

What is the ligase chain reaction test?

A

A new type of NAAT that has greater specificity than PCR

74
Q

What does the ligase chain reaction test detect?

A

-C.trachomatus and N.gonorrhoeae in urine

75
Q

What is different about identifying syphilis?

A

-Treponema pallidum can not be grown in culture in the microbiology lab

76
Q

How do you detect treponema pallidum?

A
  • Detect spirochete by direct microscopy
  • Detect specific antibodies in serum
  • Dark field microscopy using dark-field condenser looking for motile spirochetes
77
Q

What are the most useful tests for syphilis screeening?

A

VDRL and RPR

78
Q

What are VDRL and RPR measuring and what is their role in identifying syphilis?

A
  • Measure non-treponemal antibodies
  • Non specific, 1-2% of time false-positives occur
  • False negatives can happen early in disease because it takes several weeks to develop antibodies
79
Q

Once you get a positive result of syphilis in the VDRL or RPR, what is the next step to confirm results?

A

Positive results need to be confirmed with FTA-ABS or MHA-TP

80
Q

What is the FTA-ABS?

A

A fluorescent treponemal antibody absorption test to detect treponemal antibodies
-Used to confirm syphilis screening tests

81
Q

What is the MHA-TP?

A

Microhemagglutination, a serum test to dect treponemal antibodies
-Used to confirm syphilis screening tests

82
Q

What is the initial screening test to detect HIV antibodies and what happens if you get a positive test?

A

ELISA (enzyme-linked immunosorbent assay)

  • Can detect HIV-1 and HIV-2
  • High sensitivity and specificity, though false negatives can false-positives can occur
  • Need a confirmatory test for diagnosis
83
Q

What is the confirmatory test for detecting HIV antibodies?

A

-The Western blot!

84
Q

Why is the Western blot not good for screening for HIV?

A

-Lower sensitivity, but highly specific

85
Q

What are the two STIs that require both an initial screening test and a confirmatory follow up test?

A

Syphilis identification and HIV identification

Infectious Disease (8 decks)

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