INTERNATIONAL NORMALIZED RATIO FOR PROTHROMBIN TIMES IN PATIENTS TAKING ORAL ANTICOAGULANTS - CRITICAL DIFFERENCE AND PROBABILITY OF SIGNIFICANT CHANGE IN CONSECUTIVE MEASUREMENTS
Jf. Lassen et al., INTERNATIONAL NORMALIZED RATIO FOR PROTHROMBIN TIMES IN PATIENTS TAKING ORAL ANTICOAGULANTS - CRITICAL DIFFERENCE AND PROBABILITY OF SIGNIFICANT CHANGE IN CONSECUTIVE MEASUREMENTS, Clinical chemistry, 41(3), 1995, pp. 444-447
To determine when a change in serial measurements of prothrombin time
in patients receiving oral anticoagulant therapy (ACT) is a statistica
lly significant biological change necessitating dose adjustment, one m
ust know the size of the ''critical difference'' in statistical terms
(i.e., probabilities). In a cohort of 32 ACT patients at pharmacologic
al steady-state, we studied the within-subject total variation of prot
hrombin time, expressed as International Normalized Ratio (INR), over
6 months. The total within-subject variation (CV) of INR was 10.1%. Th
e corresponding critical differences required for significance of chan
ge in serial INR results was 0.7 at a therapeutic target of 2.5 INR an
d 1.0 at a therapeutic target of 3.5 INR. The data presented allow gen
eration of objective criteria for monitoring ACT patients and deciding
dose adjustments. We recommend that estimations of critical differenc
es for significant change of INR in ACT patients should be based on re
sults obtained in the specific clinic investigated to mirror the routi
ne total variation of INR measurements they obtain.