Jf. Lassen et al., INTERPRETATION OF SERIAL MEASUREMENTS OF INTERNATIONAL NORMALIZED RATIO FOR PROTHROMBIN TIMES IN MONITORING ORAL ANTICOAGULANT-THERAPY, Clinical chemistry, 41(8), 1995, pp. 1171-1176
Despite careful monitoring of oral anticoagulant treatment (OAT), some
international normalized ratio (INR) for prothrombin time values will
fall outside the therapeutic range. Considerable changes in serial IN
R results from OAT patients may be caused by random fluctuation alone,
and, for statistical reasons, a fraction of the INR values will fall
outside therapeutic range and interfere with dose adjustments. On the
basis of therapeutic intervals and statistical evaluation of reference
changes, we suggest and discuss an alternative method for interpretat
ion of serial INR measurements. Retrospective evaluation of serial mea
surements of INR from OAT patients revealed an ''overshooting'' phenom
enon. When a dose was adjusted on the basis of insignificant change in
INR value, the subsequent INR value generally fell in the opposite di
rection, If a further change of dose was initiated because of the new
INR value, a similar course in the opposite direction was observed. Th
is ''ping-pong'' effect renders patients in a fluctuating state of ant
icoagulation and may introduce increased risk of complications, The su
ggested method provides an objective criterion for dose adjustments in
OAT, which should reduce patients' risk.