Activated partial thromboplastin time (aPTT) is a coagulation screening assay that estimates the activity of the intrinsic and common coagulation pathway. The assay is used to screen for coagulation factor deficiency, monitor unfractionated heparin (UFH) therapy and detect the presence of lupus anticoagulant (LA) (1). Responsiveness of commercially available aPTT reagents on these pathophysiological conditions is different as reagents contain different types and concentrations of activators and phospholipids (2). Besides, results are highly dependent on the combination of reagent and coagulometer in use, making results comparison difficult. Therefore, it is not surprising that there is still no global assay standardization.

Activated partial thromboplastin time is determined and reported as clotting time in seconds (aPTT(s)). Reporting of the results as aPTT ratio (aPTT(r)) of patient-to-normal clotting time is often used in terms of improving comparability of the results between different laboratories, especially for UFH dose-adjustment or laboratory diagnosis of LA (3). Although it is presumed that reporting the aPTT(r) has an advantage in reducing within-laboratory variability, manufacturers usually do not provide complete information for calculation of aPTT(r) (4). What is considered as “normal” clotting time in patient-to-normal clotting time calculation usually is not defined (3, 4).

In 2005, Croatian Chamber of Medical Biochemists (CCMB) recommended reporting of aPTT as ratio, with a harmonized reference interval of 0.8-1.2 (5). As a sequel to it, in 2006, Commission for External Quality Assessment of the Croatian Society of Biochemists (predecessor of Croatian Centre for Quality Assessment in Laboratory Medicine, CROQALM), instructed laboratories to report aPTT(r) along with aPTT(s), and specified that it should be calculated by dividing the aPTT(s) result with the mean value of reference interval that laboratories use for aPTT(s) (6). CROQALM assesses performance of both, aPTT(s) and aPTT(r). Reporting of the results in both ways was also recommended in recently published national guidelines (7). According to literature, different external quality assessment (EQA) providers assess performance of aPTT in seconds, ratio or in both ways, applying quite different acceptable limits of performance, ranging from 15 to 25% (8, 9). Acceptable bias of 7% for aPTT result assessment, used by CROQALM from 2017, originates from biological variability database (10, 11). Thus, in addition to lack of standardization of aPTT assay as well as result reporting there is also no consensus on aPTT performance criteria.

The aim of study was to investigate differences in reporting aPTT results that can affect comparability of the results among Croatian laboratories and suggest further steps for its harmonization.