Considerations in the laboratory assessment of haemostasis

Abstract

Summary. This review outlines a number of key issues when performing laboratory testing of homeostasis. The effect pre-analytical variables have on the reliability and consistency of screening tests is often forgotten due to a lack of understanding and awareness. This can be improved through educating healthcare professionals who are involved in taking blood for assessment. Recent advances in coagulation testing have not enabled laboratories to replace the Prothrombin Time (PT) and Activated Partial Thromboplastin Time (APTT) screening tests with more advanced assays and they continue to play an important role with the advantage of being easily automated. However, there are many analysers on the market, each with varying sensitivity to coagulation defects and it is important to keep this in mind when interpreting results.

Recurrent thrombosis despite a therapeutic international normalised ratio

“…3 Correct sample preparation is paramount if laboratory results are to be accurate. 4,5 Perhaps this is nowhere more relevant than in coagulation testing, where inaccuracies can significantly affect patient outcome and, as demonstrated by this case, cause lifethreatening complications. 4,5 Standardisation (where possible) of pre-analytical variables minimises this risk.…”

“…4,5 Perhaps this is nowhere more relevant than in coagulation testing, where inaccuracies can significantly affect patient outcome and, as demonstrated by this case, cause lifethreatening complications. 4,5 Standardisation (where possible) of pre-analytical variables minimises this risk. Such variables include the length of time and temperature at which the sample is stored, underfilling of the tube and a raised haematocrit.…”

“…Such variables include the length of time and temperature at which the sample is stored, underfilling of the tube and a raised haematocrit. 4,5 Sodium citrate, present in coagulation tubes, works as an anticoagulant by removing the calcium ions that are crucial for coagulation reactions. 6 Different citrate concentrations in the tubes can affect coagulation screen results, with a higher citrate concentration prolonging the prothrombin time (PT) and activated partial thromboplastin time (APTT) as the ratio of citrate to calcium is increased.…”

“…6 Different citrate concentrations in the tubes can affect coagulation screen results, with a higher citrate concentration prolonging the prothrombin time (PT) and activated partial thromboplastin time (APTT) as the ratio of citrate to calcium is increased. 4,6 An elevated haematocrit, as in patients with cyanotic heart disease, 3 causes an increased proportion of red cells and a reduced amount of plasma in the collected sample. Excess citrate in these cases binds a considerable amount of the newly added calcium that is in the PT or APTT reagent, causing an artifactually prolonged clotting time and, thus, a raised INR.…”

Performance and Interpretation of Routine Coagulation Assays

“…Classically, these tests use blood collected into sodium citrate anticoagulant, which is a calcium chelator, followed by centrifugation to remove the cells and prepare platelet -poor plasma. Because calcium and phospholipid are necessary for coagulation to occur, the specimen collection and processing, which removes calcium and cells that are a source of phospholipid, therefore prevents coagulation until the addition of the appropriate reagents in the various coagulation assays [35,36].…”