Chapter 1 · Coagulation specimen integrity

Coagulation is more sensitive to pre-analytical errors than almost any other section. Key requirements: 9:1 blood to 3.2% sodium citrate (tube must be filled to the indicated line), no clots, mix gently by inversion immediately after collection, transport to laboratory within 4 hours at room temperature (not refrigerated), and centrifuge within 1 hour of collection for PT/APTT. Haematocrit-corrected citrate volume is required for patients with PCV >55% or <25%.

Chapter 1 · Practical example

Citrate tube integrity — detecting clotting before analysis

Even a small clot in a citrate tube invalidates coagulation results. Every specimen must be inspected for clots before processing.

▲ The situation

A citrate tube arrives for PT/APTT from a pre-operative ward patient. The tube appears adequately filled. During centrifugation, the plasma fails to separate cleanly.

Reference: ISO 15189:2022 Clause 7.2.7

✓
Good practice

The analyst stops centrifugation and gently inverts the tube — she feels resistance, indicating a fibrin clot. Even a small clot means coagulation factors have been partially consumed and the result will be invalid. She rejects the sample, documents the rejection, and contacts the ward explaining that a partially clotted tube produces unreliable coagulation results and requests immediate recollection with emphasis on gentle inversion immediately after collection.

✗
Poor practice

The tube is processed despite the centrifugation anomaly — the plasma looks acceptable visually. PT and APTT appear normal. The patient proceeds to surgery. Post-operatively, there is more bleeding than expected. Investigation reveals the pre-operative coagulation screen was run on a partially clotted sample — factor consumption in the tube had shortened the PT artificially, giving false reassurance.

● What this means for your practice

Inspect every citrate tube before processing — during or after centrifugation. A clot, however small, invalidates the result. The cost of a repeat collection is one venepuncture. The cost of releasing an invalid coagulation result before surgery can be catastrophic.