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SOP

Manual Platelet Count Procedure

SOP for manual platelet counting using a hemocytometer for thrombocytopenia verification and flagged automated counts.

Last verified 1 month ago 3 min read Hematology #platelets #hemocytometer #thrombocytopenia #manual count #pseudothrombocytopenia
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Document Control
Version
1.0
Effective
Aug 30, 2026
Reviewed
Aug 30, 2026
Department
Hematology
Principle

The hemocytometer method lyses red cells with ammonium oxalate so platelets can be counted in a fixed grid volume. The platelet count is proportional to the number of platelets counted in a known volume of diluted blood, so accurate dilution, representative sampling of the well-mixed tube, and correct chamber depth are critical. The method is technically demanding and operator-dependent but is the traditional confirmatory standard for low or flagged counts.

Standard Operating Procedure: Manual Platelet Count Procedure

1. Purpose and Scope

To determine the platelet count manually using a hemocytometer when the automated count is flagged (especially low counts and clumped platelet flags), when platelet clumps or giant platelets interfere, and as a confirmatory method for critical values.

2. Specimen Requirements

  • Type: Whole blood anticoagulated with EDTA (lavender-top).
  • Volume: Minimum 1.0 mL; fill to the line to avoid EDTA excess which promotes clumping.
  • Timing: Mix gently by inversion for at least 1 minute (never vigorous shaking) before sampling.
  • Rejection: Clotted sample, inadequate volume, or sample more than 4 hours old are rejected; EDTA-induced clumps require a citrate re-collection (use the same method with the citrate factor).
  • Platelets: Count promptly because platelets swell and approximate within hours.

3. Safety and Precautions

  • Standard PPE (gloves, lab coat). All blood samples are potentially infectious.
  • Dispose of lancets, pipettes, and cover glasses as sharps.
  • Do not mouth-pipette; use mechanical pipetting devices.

4. Equipment and Reagents

  • Improved Neubauer hemocytometer with cover glass (correct thickness).
  • Diluting pipettes or precision micropipettes.
  • Diluent: 1% ammonium oxalate (Unopette), which lyses red cells and preserves platelets as refractile bodies.
  • Microscope with a 40x objective.

5. Step-by-Step Procedure

  1. Gently mix the EDTA specimen for 1-2 minutes by inversion.
  2. With a precision pipette, dilute whole blood 1:20 in 1% ammonium oxalate (e.g., 10 µL blood into 190 µL diluent).
  3. Mix the dilution gently and allow 3-5 minutes for RBC lysis; platelets should remain visibly refractile.
  4. Load the hemocytometer with a small volume of the diluted suspension; allow cells to settle in a humid chamber for 10-15 minutes.
  5. Using the 40x objective, count platelets in 5 middle large (1x1 mm) squares of the central grid (one large square = 0.1 µL volume).
  6. Calculate: platelets/µL = total platelets counted x 1000 (for 1:20 dilution, 5 squares).
  7. Repeat on a second chamber; accept the mean if the two counts agree within acceptable limits (within 10-15%).
  8. Document clumping, giant platelets, and any red cell fragments that interfere.

6. Quality Control

  • Verify the hemocytometer grid and cover glass depth at regular intervals.
  • Count exactly 5 squares consistently; record the dilution and arithmetic on the worksheet.
  • Compare manual results periodically with the automated analyzer; investigate persistent discrepancies beyond 15%.
  • Duplicate counts by a second technologist at least weekly as a competency check and for critical results.

7. Decision limits

The interpretive thresholds below are clinical decision limits; the canonical adult reference interval is displayed in the Reference Ranges panel.

  • Thrombocytopenia: platelet count below 150 x10^3/µL. Mild thrombocytopenia (below 150) usually poses low bleeding risk; risk rises progressively below 50 x10^3/µL (institution-defined critical threshold, commonly 50 but some laboratories use 20 x10^3/µL for the critical value).
  • Critical value: most institutions answer a critical call below 20-50 x10^3/µL because spontaneous bleeding risk increases substantially.
  • EDTA-induced pseudothrombocytopenia: if clumping is seen and the count is paradoxically low, re-collect in a citrate tube and repeat; rechecking with a blood film is mandatory before reporting a low manual count.
  • Quality check on the film: the film estimate (1 platelet per oil-immersion field x 20,000 = approximate count) should broadly agree with the chamber count.

Reference Ranges

Browse all ranges
Platelets - Adult

Adult · Any

150–400 ×10³/µL

Critical values apply

Frequently Asked Questions

EDTA-induced platelet clumping (pseudothrombocytopenia) is the classic cause; clumps are counted as cells or ignored. Manual review with a blood film, or re-collection in citrate, resolves the discrepancy.
Whole blood is diluted 1:20 in 1% ammonium oxalate, and platelets are counted in 5 middle large squares (each 0.1 µL under the 1:20 dilution); an accepted dilution/count yields the count multiplied by 1000.
When the automated result is flagged low, suspected platelet clumps are present, the instrument gives a critical value, or the film appearance disagrees with the histogram, a manual confirmatory count is required before the result is released.