August 5, 2015 - Laboratory Diagnosis of Infectious Diseases

Question 1

Minimum Inhibitory Concentration (MIC)

Answer 1

The minimum concentration of an antibiotic that is required to inhibit microbial growth. This is determined by serial dilution susceptibility testing.

This is used in the lab to determine if the pathogens are susceptible, intermediate, or resistant to different antibiotics.

Question 2

Growing Fungal Culture

Answer 2

Often takes much longer to grow than bacteria. Most common bacteria will grow in a few days, but fungal and mycobacterium can take weeks.

Question 3

Viral Culture

Answer 3

This is being used less and less for diagnostic purposes, but can be done by exposing a layer of cells to the virus. It will lyse the cells and create holes.

Question 4

Pros and Cons of Culture

Answer 4

The good:

• Very specific - if a pathogen is there, it is likely causing infection
• Can definitively identify the organisms
• Can do susceptibility testing

The bad:

• Sensitivity is variable
• Slow

Question 5

Pros and Cons of Microscopy

Answer 5

The good:

• Very quick (minutes to hours)
• Inexpensive and easy
• Can guide empiric therapy while waiting for culture

The bad:

Sensitivty is lower: lower quantities of organisms may not be seen

• Can’t directly identify/speciate the organisms
• Only as good as the expert reading it

Question 6

Window Period

Answer 6

The time period where the patient is infected with the organism, but antibody levels are not developed to detectable levels.

Question 7

Pros and Cons of Serology

Answer 7

The good:

• Can be sensitive and specific
• Easy, quick, and inexpensive

The bad:

• Can have cross-reactions (false positives)
• Window periods can interfere with results