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David R. Anderson, MD; Linda Harrison, RN; Jack Hirsh, MD

Arch Intern Med. 1993;153(12):1441-1447.

Abstract
Background
The anticoagulant activity of warfarin sodium is monitored by the prothrombin time (PT). The introduction of a portable PT monitor has raised the possibility that patients could reduce the inconvenience of anticoagulant therapy by measuring their PT at home. We performed this study to determine the feasibility and accuracy of home use of the portable PT monitor.

Methods
A prospective cohort study was performed in consecutive eligible patients who required long-term anticoagulant therapy. Patients performed multiple measurements of their PT at home by means of the portable monitor and at their usual laboratory within a 4-hour interval. The accuracy of the portable monitor was evaluated by two criteria for agreement. Standard agreement was achieved if the portable monitor and laboratory results were both either within or outside the patient's targeted therapeutic range or if the two results were within 0.4 international normalized ratio units of each other. Expanded agreement was achieved if both the portable monitor and laboratory results were within ±0.4 international normalized ratio units of the targeted therapeutic range.

Results
Forty patients (19 men and 21 women, aged 25 to 74 years) were followed up for 6 to 24 months by means of the portable PT monitor. The mean level of agreement achieved per patient was 83% (95% confidence interval, 79% to 87%) by the standard criteria and 96% (95% confidence interval, 94% to 98%) by the expanded criteria. Twenty-seven patients (68%) and 39 patients (98%) achieved more than 80% agreement by the standard and the expanded criteria, respectively. Questionnaire results revealed that 97% of the patients preferred using the portable monitor to measure their PT.

Conclusions
Patients receiving long-term anticoagulant therapy achieved a high rate of clinically important agreement between self-measurements of the PT with the use of a portable monitor and laboratory PT results. Patients strongly preferred using the portable monitor to measure their PT levels. The use of the portable monitor as the primary method for measuring the PT can be recommended in selected patients receiving long-term anticoagulant treatment.

(Arch Intern Med. 1993;153:1441-1447)

Author Affiliations
From the Hamilton (Ontario) Civic Hospitals Research Centre and the Department of Medicine, McMaster University, Hamilton.; Dr Anderson is now with Victoria General Hospital, Halifax, Nova Scotia.

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