Chapter 3 · Coagulation

Coagulation testing (PT, INR, APTT, fibrinogen, D-dimer) requires specific pre-analytical conditions: 9:1 ratio of blood to sodium citrate, full tube, no clots, transport at room temperature within defined time limits. Citrate concentration (3.2% vs 3.8%) affects INR — the laboratory must document which it uses. INR critical values (typically >5.0) require immediate notification. Lupus anticoagulant testing requires specialised specimen handling.

Chapter 3 · Practical example

Platelet clumping — pseudothrombocytopenia and blood film confirmation

EDTA-induced platelet clumping causes falsely low platelet counts. Always make a blood film to confirm isolated thrombocytopenia before releasing a critical low platelet result.

▲ The situation

A routine FBC on an asymptomatic patient shows platelets 24 x10^9/L. The patient has no bleeding history and takes no antiplatelet medication. There is no previous count on record.

Reference: ISO 15189:2022 Clause 7.3.7; ICSH Platelet Guidelines

✓
Good practice

The analyst makes a blood film and scans the feather edge under low power: abundant platelet aggregates are visible across the smear. This is EDTA-induced pseudothrombocytopenia. She contacts the requesting clinician, explains the finding, and requests recollection in a citrate tube processed within 30 minutes. The citrate result shows platelets 178 x10^9/L — entirely normal. The original EDTA result is annotated accordingly.

✗
Poor practice

The platelet count of 24 triggers a critical value notification. The clinician urgently refers the patient to haematology. The haematologist requests a repeat FBC — platelets 182 x10^9/L in a correctly collected citrate tube. The patient has attended an urgent clinic, undergone stress, and potentially been given dietary restrictions based entirely on a laboratory artefact.

● What this means for your practice

Any unexplained isolated thrombocytopenia in a clinically well patient requires a blood film examination before the result is released. Platelet clumps on the film are the laboratory’s responsibility to identify. The clinical consequences of undetected pseudothrombocytopenia can include unnecessary treatment and patient harm.