The degree of inhibition will depend on the treatment intensity and the age of the child. For this reason, warfarin therapy is rarely recommended for infants less than 2 months of age. By 6 months of age, the coagulation system is similar to the adult system and warfarin therapy can be used with careful monitoring. Monitoring General guidelines for monitoring warfarin therapy in children are similar to adult patients. By using the INR, monitoring of warfarin is simplified and its safety is improved by the standardization of the therapeutic range, irrespective of the thromboplastin reagent used.
Many laboratories now report INRs.
The ISI is the International Sensitivity Index for a given reagent. The mean of the normal range should be calculated by obtaining the mean of a minimum of 20 subjects' PTs with the PT determined using the laboratory's individual thromboplastin. This calculation is done by the laboratory. Since the half lives of the vitamin K-dependent coagulation factors vary from 6 to 72 hrs and the half life of warfarin is 2.5 days, changes made to the dosage will not be reflected in the INR value completely until day 3 or 4. Maintenance doses are highly dependent on patient age, vitamin K intake, intercurrent illnesses and concurrent use of other drugs.