Figure 20.9 Neubauer hemocytometer counting chamber. Note the difference in depth depending on area examined.

312 PartV • Laboratory Procedures

• If 25 squares of middle square are counted (if the platelet count is < 100,000, count all 25 squares of the middle square):

Multiply No. of platelets X 1000 = No. of platelets/mm3


150,000 to 410,000 platelets/mm3


1. Specimen should be properly mixed and have sufficient volume of blood so there is no dilution of anticoagulant.

2. The capillary tube must be filled completely and be free of any air bubbles.

3. After the hematocytometer is charged, it should be placed in a premoistened Petri dish to prevent evaporation while the cells are settling out.

4. The light adjustment is critical. It is important for both WBCs and especially platelets. If the condenser is not in the correct position, it will fade out platelets.

5. Debris and bacteria can be mistaken for platelets.

6. Clumped platelets cannot be counted properly; the specimen must be recollected. The anticoagulant of choice is EDTA for preventing platelet clumping.

7. Avoiding overloading of hemacytometer chamber.

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