What is the test for manual platelet count?

Manual Platelet Counting Methods

The manual platelet count is performed using phase-contrast microscopy with a hemocytometer (Neubauer chamber), which remains the gold standard reference method for platelet enumeration. 1, 2

Primary Manual Counting Technique

Phase-contrast microscopy using a hemocytometer is the established manual method for platelet counting, though it has been largely replaced by automated techniques in routine practice due to being time-consuming and having high imprecision. 1, 2

Technical Specifications

• The Neubauer hemocytometer is used for direct platelet counting, where platelets are uniformly distributed across rows, columns, and sides of the chamber 3
• Counting any 10 squares on either side of the hemocytometer and multiplying by a constant factor can accurately predict the total platelet count, reducing counting time and fatigue-related errors 3
• This manual phase-contrast method was historically used by manufacturers to calibrate automated cell counters and quality control materials 2

Alternative Manual Methods

Blood Smear Estimation Methods

Two validated blood smear techniques exist as alternatives to hemocytometer counting:

• Platelet count based on white blood cells (PCW): Count platelets per white blood cell on a stained blood smear, then calculate using the automated WBC count 4

  • This method shows excellent correlation with phase-contrast microscopy (r² = 0.99) 4
  • Maximum deviation is only 2 × 10⁹/L for counts <20 × 10⁹/L 4
  • Superior to automated impedance methods in thrombocytopenia, which can overcount by up to 37 × 10⁹/L 4

• Red cell to platelet ratio method: Count platelets per 1000 erythrocytes, then multiply by the automated RBC count 5

  • Shows strong correlation with automated methods (r = 0.908, ICC = 0.905) 5
  • 93% of differences fall within agreement limits, with 77% having differences <20,000 platelets/µL 5

Key Advantages of Blood Smear Methods

• Visual discrimination between true platelets and non-platelet particles (debris, RBC fragments, blast cell fragments) that confound automated counters 4
• Identification of platelets with abnormal size that automated methods may miss 4
• Morphological clues to thrombocytopenia etiology (leukemia, infection, HUS, familial macrothrombocytopenia, immune thrombocytopenia) 4
• Faster and requires less technical expertise than phase-contrast microscopy 4

Clinical Context

Manual platelet counting becomes essential when automated counters fail in thrombocytopenia, as most automated impedance-based counters cannot reliably discriminate platelets from similar-sized particles in these situations. 4 The blood smear methods are particularly valuable for rapid clinical decision-making when platelet counts are critically low (<20 × 10⁹/L). 4