By Levi Clancy for Student Reader on
Assays to diagnose infectious disease must be: specific (no false positives or cross reactivity); and sensitive (able to identify even a single cell). Indirect assays detect antibodies, which are evidence of past and perhaps prsent infection. Direct assays detect the parasite itself.
PCR detection of any microbe has two criteria: specificity to detect only the microbe in question; and sensitivity to detect the microbe at low abundance.
Diagnostic microscopy is usually performed on blood smears or following culture in synthetic media. Centrifugation can be performed to concentrate scarce parasites into a white cell layer called the buffycoat.