For which of the following diagnostics is low-power magnification on the microscope not used?

• A. To detect rouleaux
• B. To estimate platelet numbers
• C. To detect RBC agglutination
• D. To detect clumping of platelets

Answer: (B)

Understanding how magnification levels guide what we observe on a blood smear helps here. Low-power viewing is used to screen the overall smear for patterns among many cells: rouleaux appear as stacked coin-like formations, RBCs may show agglutination (clumping), and platelets can appear as small clumps or irregular associations with red cells. These broad patterns are easiest to detect when you’re looking at many fields at once.

Estimating platelet numbers, on the other hand, requires resolving individual platelets clearly, which means using high power (often oil immersion). At low power, platelets are too small and numerous to count accurately, so this task is not performed with low-magnification viewing.

So, the activity that does not use low-power magnification is estimating platelet numbers.