The importance of commutability in material used for quality control purposes

External Quality Assessment (EQA) is an important part of laboratory quality assurance. Spiking of normal plasma is sometimes employed to mimic clinical samples. It is important that spiked material gives similar results to clinical samples (ie, is commutable) to ensure appropriate conclusions can be drawn from EQA exercises. We describe here data from UK National External Quality Assessment Scheme for Blood Coagulation (NEQAS BC) exercises where spiked samples were tested alongside samples from patients to explore commutability of artificial material. Normal plasma was spiked with unfractionated heparin (UFH), low molecular weight heparin (LMWH), Dabigatran or Rivaroxaban. Factor VIII (FVIII) deficient plasma was spiked with FVIII concentrate. Spiked samples and ex vivo patient material were sent to laboratories for testing. For LMWH, good agreement was seen between results for samples from patient plasma and plasma spiked with heparin. For UFH, APTT ratios differed between spiked and patient samples for the same drug concentration, with no correlation in ranking of reagents. Precision for patient and spiked material for Rivaroxaban and Dabigatran assays was comparable. However, the pattern of results for some Dabigatran assay kits differed between spiked and patient samples. For FVIII assays, results obtained with spiked and postinfusion samples gave comparable results. Spiked material is suitable for EQA if commutability is demonstrated. Our data show commutability for plasma spiked with Rivaroxaban, LWMH and some FVIII concentrates. For some tests, notably APTT for UFH, marked differences between patient and spiked samples indicate not all tests can be evaluated using artificial samples.