Laboratory Control of Coumarin Therapy

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Abstract
The Quick "prothrombin test" is widely used as the guide for determining dose of coumarin to be prescribed. It is a bio-assay which requires meticulous technique, experienced technicians and reliable reagents. More than one laboratory may be involved in the care of an individual patient, since treatment usually begins in hospital during the acute illness, then continues as an outpatient. To determine the validity, reproducibility and reliability of the test, and to compare results in different laboratories, bloods were drawn from 30 patients in various stages of coumarin treatment, 45 cc. of blood being added to 5 cc. oxalate, centrifuged, the plasma decanted, mixed, divided into 0.7 cc. aliquots, one-half of each plasma coded one number, the other half another number, and then distributed to each of 13 cooperating laboratories, to be tested and reported by usual techniques. Commercially available control plasmas and uniform thromboplastins were similarly distributed. The results were widely divergent, not only in comparing the laboratories one to another, but also within the same laboratory. The main causes of the discrepancies were considered to be in the diluent curves from which the "percentage" is determined, in the diluent and techniques of making the dilution curves, various types of thromboplastin being used, modifications in technique, technician errors, prolonged time in and differences in temperature of the water bath. Proposals designed to correct these discrepancies include elimination of the dilution curves, reporting results as seconds and ratio, adoption of a uniform, meticulous technique, training and frequent checking of technicians, standard glassware, temperature control, uniform thromboplastin, anticoagulant control laboratory, and periodic testing of unknown plasmas.
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