Underestimation of plasma level of factor V coagulant activity and fibrinogen concentration together with prolonged prothrombin time, activated partial thromboplastin time and thrombin time can result from pre-analytical very low calcium level in citrated sample tube.

Introduction: Pre-analytical phase is a critical step in the haemostasis laboratory cycle. Numerous variations affect tests results, and it is crucial to detect them in order to reject improper specimens before reporting test results. Comparing to prior results or requesting, a repeat sample can help in pre-analytical irregularity assessment.

Methods: Each time a sample addressed to our laboratory displayed aberrant results or discordant with a prior report, another specimen was asked and both were analysed through calcium (Ca) level, prothrombin time (PT), activated partial thromboplastin time (APTT), thrombin time (TT), fibrinogen concentration, factor II, factor VII+X and factor V coagulant activity measurements. Among these, all the primary citrated samples from inpatients without anticoagulant treatment, displaying very low calcium level ('Ca 0' samples), were selected for this 2 years study.

Results: A total of 17 samples could be identified. Ca level in their paired repeat samples was always >1.00 mmol/L. Coagulation testing for 'Ca 0' samples showed a significant prolongation of PT, APTT, TT and a significant decrease for fibrinogen concentration and factor V coagulant activity.

Conclusion: We identified factor V coagulant activity, as the parameter with the most important variation in case of very low calcium level in presumed citrated sample tubes probably contaminated with EDTA.