Chapter 3 · Common coagulation tests and interpretation

PT/INR: used for monitoring warfarin therapy and assessing liver function. APTT: screens for intrinsic pathway defects and monitors unfractionated heparin. Fibrinogen: acute phase reactant and haemostasis marker — critically low in DIC. D-dimer: highly sensitive but non-specific for VTE; elevated in many conditions. Mixing studies: used to differentiate factor deficiency (corrects) from inhibitor (does not correct).

Chapter 3 · Practical example

APTT for heparin monitoring — using the right therapeutic range

APTT therapeutic ranges for unfractionated heparin monitoring are reagent-specific. Using published generic ratios without local validation is a systematic error.

▲ The situation

A patient on unfractionated heparin for acute pulmonary embolism has an APTT ratio of 2.1. The haematologist asks whether the patient is in the therapeutic range.

Reference: ISO 15189:2022 Clause 7.3.5; BSH Heparin Guidelines

✓
Good practice

The coagulation scientist checks the laboratory’s validated heparin nomogram — established by correlation with anti-Xa assay using this laboratory’s specific reagent and analyser. For this system, the therapeutic ratio range is 1.8 to 3.2 for a target anti-Xa of 0.3 to 0.7 U/mL. APTT ratio 2.1 corresponds to anti-Xa approximately 0.38 U/mL — within therapeutic range. This is reported with the anti-Xa equivalent.

✗
Poor practice

The scientist uses the commonly cited therapeutic APTT ratio of 1.5 to 2.5 from a published guideline. Ratio 2.1 is within this range — reported as therapeutic. However, this laboratory’s reagent is sensitive to heparin. The actual therapeutic range for this system is 2.2 to 3.5. APTT ratio 2.1 in this system corresponds to anti-Xa 0.22 — subtherapeutic. The patient is under-anticoagulated and at risk of VTE recurrence.

● What this means for your practice

APTT therapeutic ranges for heparin monitoring must be validated for your specific reagent-analyser combination and correlated with anti-Xa assay. Published ratios are starting points for other laboratories’ systems — not defaults for yours.