Which cytology technique is used to detect certain parasites in blood samples, such as microfilariae?

• A. Wet prep
• B. Squash prep
• C. Willis prep
• D. Modified knott prep

Answer: (D)

Detecting microfilariae in blood relies on concentrating the parasites so they’re easier to see under the microscope. The Modified Knott prep does exactly that: it mixes a small blood sample with a formalin-containing solution to lyse red blood cells, then centrifuges the mixture so the microfilariae gather in the sediment. Examining this concentrated sediment makes it much more likely to detect parasites that are present in low numbers, which is common with filarial infections. A simple wet prep looks at a drop of blood for motile organisms but isn’t sensitive enough for sparse microfilariae. Squash prep and other cytology methods aren’t designed to concentrate circulating parasites from blood, so they’re less effective for this purpose. So the Modified Knott prep is the best method for detecting microfilariae in blood.