1-43 Laboratory Diagnosis of Infectious Diseases

Question 1

Microscopy

Sensitivity
Specificity
Trunaround Time

Answer 1

good for many microbes, some will be missed due to small size

knowlodgeable clinican can combine microscopy results with clinical knowled

turnaround time - fast, gram staining and acid fast staining are fast

Question 2

culturing

sensitivty
specificity
turnaround time

Answer 2

• Sensitivity: Good, though for some microbes, special media is required to grow them. Some microbes cannot be cultured at all (e.g. M. leprae).
• Specificity: Can tell you what kind of metabolites the organism uses based on what kind of media it grows on. Colony characteristics (size, topography, opacity, etc.) also help narrow down the kinds of possible microbes.
• Turnaround time: In many cases takes four days, but largely depends on how long the organism takes to grow. With TB, for example, it takes up to eight weeks to get results.

Question 3

bacterial antigen detection

sensitivty
specificity
turnaround time

Answer 3

Sensitivity: Depends on the individual organism, how it was collected, and how it was transported, among other factors.

Specificity: Immunoassays performed (e.g. ELISA) are generally 100% specific.

Turnaround time: Good, as there is no need to wait for the specimens to grow.

Question 4

Serological Diagnosis –

Answer 4

• Sensitivity: Good, as sample collection is easy.
• Specificity: Since it works off of the principles of acquired immunity, it is essentially 100% specific.
• Turnaround time: If the infection has a short incubation period, there can be a 2-3 week delay.

Question 5

Molecular Assays –

Answer 5

• Sensitivity: Great, as PCR is used to amplify obtained genetic material.
• Specificity: Good specificity, as it is DNA-based.
• Turnaround time: Good; only have to wait for the PCR and annealing of tagged probes.

Question 6

antigen detection advantage and disadvantage

Answer 6

Advantages: Generally 100% specific, some assays are very easy and convenient (done directly from a tonsillar area swab)

Disadvantages: Sensitivity is largely dependent on how the specimen was collected or transported

Question 7

molecular assay advantage and disadvantage

Answer 7

very sensitive, very specific

Advantages: Very sensitive, very specific

Disadvantages: Relatively expensive, risk of contamination and false-positives, neither 100% sensitive nor 100% specific

Question 8

typical 4-5 day testing schedule

Answer 8

Day 1 - plated on appropriate media, do staining
Day 2 - look at culture plates - identify and set up sensitivity
Day 3 - iD and sensitivity tests read
Day 4 - physician review of culture results, or sometimes day 5

Question 9

five general approaches of testing

Answer 9

• microscopy - gram stain can be clear indicator before you get culture results
• culture
• detection of bacterial antigens
• demonstration f specific nucleic acids (molecular techniques)
• detection of antibodies direct against organism (serology) - we depend on this for certain infections, have to depend on IgG and IgM

Question 10

gram + cocci clusters

Answer 10

staph

Question 11

gram + cocci chains

Answer 11

strep