The use of various types of indwelling catheters and needles for monitoring physiological parameters and for administration of intravenous fluids and medication is now widespread. Such catheters also provide a ready means of obtaining blood samples. Far too little attention, however, has been paid to errors that may arise in tests performed on blood samples obtained from patients with venous or arterial catheters in place. Instances of "nondisease" are thus inadvertently created, and much unnecessary time and effort may be expended before the error is recognized.
The prolongation of the activated partial thromboplastin time (aPTT) by heparin is a case in point. Of the routine clotting tests generally available, the aPTT is particularly sensitive to heparin, a fact still poorly appreciated by many physicians. The degree of prolongation of the aPTT by heparin is difficult to predict and depends on the amount of heparin infused, the time interval between infusion
Czapek EE. latrogenic Prolonged aPTT: A Nondisease State. JAMA. 1974;227(11):1304. doi:10.1001/jama.1974.03230240062031
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