Diagnostic Microbiology and Laboratory Methods
What you must know
Diagnostics questions test specimen selection, transport, culture, staining, biochemical identification, serology, and molecular methods. Know that culture remains the reference for bacterial isolation but is slow and may fail with prior antimicrobial therapy; PCR offers speed and sensitivity but does not distinguish live from dead organisms or colonization from infection. Understand Gram stain interpretation, acid-fast staining for mycobacteria, darkfield for spirochetes, cytology for fungal elements, and virus isolation, antigen detection, and serology (paired sera for rising titers). Know quality control, antibiogram use, and biosafety considerations.
Traps
Questions love timing of sample collection (before antimicrobials) and paired sera (acute and convalescent) for serologic diagnosis. Another trap is assuming a positive PCR always means active infection - it may reflect colonization, vaccination (some vaccines), or persistence. Watch for questions that confuse sensitivity with specificity or that treat culture as always superior to PCR. Also beware of transport media mismatches (e.g., viral transport vs. bacterial swabs).
Rule of thumb
If the stem asks how to confirm recent infection serologically, look for paired sera showing a rising titer; a single titer is often uninterpretable.
Worked example
A question describes a dog with respiratory signs treated with antibiotics before sampling, and asks why culture is negative. The answer is prior antimicrobial therapy can suppress growth; PCR may still detect nucleic acid. The discriminator is timing of sampling relative to treatment.