Lab Diagnosis of Infectious Disease

Question 1

1. List and discuss the points to consider when doing a history and physical exam for an infectious disease.

Answer 1

timing and nature of fevers 
contact with others who are ill
predisposing factors
hx. of recent or recurrent infection
travel hx.
animal contacts
recent or current antimicrobial therapy

temperature
search for local or generalized lymphadenopathy
the skin
exam of each organs system

Question 2

2. Describe the general principles of specimen collection.

Answer 2

appropriate container
correct site sampled before antibiotics, adequate volume
appropriate media, seal and transportation (i.e.. timely, anaerobic concerns)
appropriate storage
complete labeling and information
mark high risk samples

collection should bypass areas containing normal microbiota
collection during acute phase

Question 3

3. Discuss the terms sensitivity and specificity as they relate to laboratory tests.

Answer 3

sensitivity: probability that test will be positive int eh presence of a pathogen/disease
specificity: probability that a test will be negative if the pathogen or disease is not present

Question 4

4. Explain the microscopic methods used to diagnose infectious processes.

Answer 4

morphology of bacteria is too simple by light microscopy alone

1. Gram stain can tell you which morphological group and help with selection of antibiotics, but is less useful in areas with normal microbiota (mouth or fecal samples)
2. Acid-fast strain/ Ziehl-Neelsen stain used to identify mycobacteria (TB): lipids and my colic acids retain the initial red stain
3. KOH prep: looks for yeast and other fungal forms, after fixing slide and tx. with KOH, only fungal elements are spared, slide examined under low light
4. Wet preparations: directly observe a liquid sample under microscope; fluorescent, phase contrast and electron microscopy can also be used

Question 5

5. Describe the use of culture in identifying the etiologic agents causing infectious disease, including their advantages and limitations.

Answer 5

most bacterial cultures are incubated at 35-37deg. C for 24-48 hrs
solid culture media are inoculated with a wire loop, after 18-24hrs. colonies are usually visible, culture can be used for identification and susceptibility with different mediums; the colonial morphology and hemolytic patterns can also be helpful
selective medias can be used to inhibit the growth of the normal microbiota, and to select for groups of bacteria
differential media are used to distinguish between groups of organisms based on specific biochemical reaction

cultures require interpretation by physician

Question 6

5. Describe the use of antibody response in identifying the etiologic agents causing infectious disease, including their advantages and limitations.

Answer 6

useful in measuring humoral immune response when pathogen is difficult or impossible to isolate or grow by culture (ie. ELISA and Western blot)

IgM (first encounter, can’t cross the placenta) and IgG (long term infection can cross placenta

baseline and later antibody samples are compared, 4x increase in antibodies is diagnostic

Question 7

5. Describe the use of antigen detection in identifying the etiologic agents causing infectious disease, including their advantages and limitations.

Answer 7

benefits: rapid diagnosis, greater sensitivity in some cases, DNA can still be detected after negative culture, ID of organisms that are difficult to grow
limitations: inability to perform further tests, separate test for each suspected pathogen

direct fluorescent antibody test used with Chlamydia and Treponema
PCR can detect small amounts of material (MRSA, Chlamydia and gonorrhoeae, mycobacteria and virus detection- CMV, HSV, HBV, HIV)

Question 8

6. List the characteristics of the 4 organisms presented in table 2. (4)

Answer 8

characteristics that are useful in differential growth/ metabolic testing include:

growth on EMB
lactose fermenter/ non lactose fermenter
produces urease
produces oxidase
other distinguishing characteristics

Question 9

Which to bacteria still remain 100% susceptible to penicillin?

Answer 9

Streptococcus progenies and Treponema palladium