Lab Diagnosis of Bacterial Infections

Question 1

Which of the following is not a diagnostic technique used to identify the cause of infections: microscopy, culture, antigen detection, karyotyping, serology

Answer 1

Karyotyping. The technique not listed is nucleic acid assays / molecular testing.

Question 2

The Gram stain is an example of what diagnostic technique: microscopy, culture, antigen detection, molecular tests, or serology?

Answer 2

Microscopy

Question 3

What is the greatest advantage of using microscopic methods for detecting infection: sensitivity, specificity, or turnaround time?

Answer 3

Turnaround time: microscopy is rapid, but generally nonspecific and insensitive

Question 4

What are the approximate turnaround times for culturing: common bacteria, fastidious bacteria, fungi, and mycobacteria?

Answer 4

common: 3-4 days, fastidious: 3 weeks; fungi: 5 weeks; mycobacteria: 8 weeks

Question 5

When ordering a culture, why is it important to know which organisms are routine and which are more unusual?

Answer 5

The more unusual organisms may need specialized media or other specific conditions in order to grow.

Question 6

Latex agglutination, coagglutination, direct fluorescent antibody, and ELISA are all examples of what diagnostic technique: microscopy, culture, antigen detection, molecular tests, or serology?

Answer 6

Antigen detection

Question 7

In terms of turnaround time, sensitivity, and specificity, what are some advantages and disadvantages of using antigen detection methods?

Answer 7

Pro: turnaround time is fast, specificity is very good. Con: poor sensitivity (ex. meningitis antigen tests are no better than Gram stains for sensitivity)

Question 8

Which diagnostic technique can been used to identify bacteria that are hard to grow in vitro, to determine disease susceptibility, and to look for a past infection: microscopy, culture, antigen detection, molecular tests, or serology?

Answer 8

Serology (antibody detection)

Question 9

In terms of turnaround time, sensitivity, and specificity, serology lags at least 1-2 weeks behind the onset of infection and is associated with both high false-positives and false-negatives. Why then do we even use this technique?

Answer 9

Although it is not perfect, serology tests are widely available, samples of blood are easy to obtain and transport, and they are useful for confirming other diagnostic tests.

Question 10

A good way to use serology is to compare IgG antibodies during what 2 phases? What kind of result is considered diagnostically significant?

Answer 10

Acute phase vs. Convalescent phase, collected 2-4 weeks apart: a 4-fold rise in titer verifies recent infection

Question 11

This is a chart of possible results after testing for 4 different Hepatitis B antibodies. What methods of diagnosis are being combined here, and how would you describe a patient that has both HepB specific IgM and sAg (surface antigen)?

Answer 11

This is an example of antigen detection plus serology. A patient who is positive for both IgM and sAg has an acute HBV infection.

Question 12

What method does the Red Cross use to screen for infections in blood donations: microscopy, culture, antigen detection, molecular tests, or serology?

Answer 12

Molecular tests

Question 13

Molecular diagnostic tests are likely to be used for agents with what kinds of characteristics?

Answer 13

Agents that are slow, difficult, or dangerous to grow

Question 14

What type of diagnostic test is best to use for the detection of agents involved in encephalitis, of viral load in the bloodstream, and of antimicrobial resistance genes: microscopy, culture, antigen detection, molecular tests, or serology?

Answer 14

Molecular tests

Question 15

Nonamplified nucleic acid assays generally use hybridization to detect what type of molecule?

Answer 15

Ribosomal RNA

Question 16

Amplified nucleic acid assays generally use what method to create amplicons of a specific area of the genome?

Answer 16

PCR

Question 17

Describe nucleic acid based molecular tests in terms of turnaround time, sensitivity, and specificity. Do amplified or non-amplified assays have better sensivity and specificity?

Answer 17

Turnaround time is fast, sensitivity and specificity are extremely good. Nonamplified assays have better sensitivity and speciificity (amplified reactions are more often contaminated by amplicons or amplification inhibitors)

Question 18

Although molecular testing has very good turnaround times, sensitivity, and specificity, there are still some limitations to these assays. List 2 such limitations.

Answer 18

Expensive, not widely available

Question 19

If a Gram stain for bacterial meningitis shows Gram(+) cocci in clusters, what is the mostly likely organism?

Answer 19

Staphlococcus

Question 20

If a Gram stain for bacterial meningitis shows Gram(+) cocci in chains or pairs, what is the mostly likely organism?

Answer 20

Streptococcus

Question 21

If a Gram stain for bacterial meningitis shows Gram(+) rods, what is the mostly likely organism?

Answer 21

Listeria monocytogenes

Question 22

If a Gram stain for bacterial meningitis shows Gram(-) rods, what is the mostly likely organism?

Answer 22

Escherichia coli

Question 23

If a Gram stain for bacterial meningitis shows Gram(-) diplococci, what is the mostly likely organism?

Answer 23

Neisseria meningitidis

Question 24

The gold standard for detecting Legionella pneumophila uses what diagnostic technique and what kind of tissue sample?

Answer 24

Antigen detection in a urine sample

Question 25

What is the best laboratory technique for detecting Herpes Simplex Encephalitis?

Answer 25

Molecular assay: CSF PCR

Question 26

What common cause of pharyngitis can be detected using a antigen rapid diagnosis gadget or when it produces beta hemolysis on sheep blood agar?

Answer 26

Strep pyogenes (Group A Streptococcus/GAS)