POC monitors intended primarily for warfarin management (Table 3.2) provide whole blood PT/INR testing, and are used primarily for clinic and home monitoring. PT/INR monitors employ unit-use reagent strips that contain thromboplastin reagents that are assigned an International Sensitivity Index (ISI) based on a comparison between a whole blood device and a reference plasma method. Theoretically, this indirect assignment (plasma to whole blood) could present difficulty in comparing results between plasma and whole blood tests using reagents with comparable ISI assignments. However, comparability is generally quite good when the ISI values are similar [4]. The preferred specimen type is fresh capillary whole blood, though untreated venous blood collected in a plastic syringe is also acceptable for some analyzers. These instruments detect clot formation through electrochemical (impedance) or mechanical (cessation of movement of iron particles; change in rate of blood/reagent mixture movement) methods (Table 3.2). Patients who have tested positive for a lupus anticoagulant should not be monitored with these devices due to the potential for interference with the test. In general, patients who are being treated with heparin should also not be monitored with these devices. However, the Roche Coaguchek assays incorporate heparinase into the test and can neutralize low dose heparin therapy.

Published comparisons of INR values produced by PT monitors and laboratory coagulation analyzers on the same specimens generally demonstrate good agreement in the therapeutic range [4]. POC INR values above 3.0 often have a positive bias relative to the laboratory instrument value, and the degree and spread of disparity usually increases with increasing POC INR. This positive bias may be more common in patients who are not stably anticoagulated on warfarin (e.g., new patients, noncompliant patients) due to differential sensitivities of recombinant versus tissue-extract thromboplastins to FVII levels [5]. Some clinicians and/or laboratories may decide to confirm POC INR values by the laboratory method at values above an INR of 4.0. This approach seems prudent since the laboratory analyzer is expected to be more precise, and the POC value is generally more susceptible to sample collection error. However, published data support the safety of managing patients solely by POC INR [4, 6–8]. Nonetheless, POC coagulation analyzers should be validated against the laboratory reference method, and clinicians should be made aware of the analytical relationship between the methods, since patients may be tested by different modalities during the course of their routine care. In addition, periodic comparisons against the laboratory reference method are advisable [9].

Prior generations of waived instruments lacked the capability for automatic download of results into an information system. This posed a significant risk for transcription errors. Fortunately, most waived POC PT/INR instruments now have download capability.

The primary clinical application of these instruments is real time monitoring of temporary heparinization during invasive procedures. They may also be used to direct fresh frozen plasma transfusion therapy during surgical procedures. Real-time heparin management is employed in a variety of settings including cardiac bypass surgery, percutaneous coronary therapeutic interventions, radiological procedures and extracorporeal membrane oxygenation. Instruments used to monitor heparin are generally less specialized than the PT monitors, frequently offering the PT, the PTT and the activated clotting time ACT tests on the same device. Fresh whole blood is the predominant sample employed by these analyzers. They detect clot formation by mechanical methods (detection of cessation of movement of fluid; detection of displacement or rotation of a magnet; measurement of resistance to movement of a plunger through the sample), and in one case, by electrochemical (amperometric) detection (Table 3.3).