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What is the difference between the APTT and INR labs? APTT is used for heparin, and INR for coumadin monitoring, correct? Is there anything else?
INR (International Normalized Ratio) is a standard recommended by the World Health Organization (WHO) that applies to PT. Basically, it standardizes the value for PT despite the reagent used for the test. Some theorize that INR is somewhat of a calculated value--regardless of that since INR is correlated with PT and PT will not change with heparin admin...you are on the right track. When giving heparin the lab test of interest will be PTT, APTT, or w/ high doses of heparin during, say, bypass surgery, ACT (activated clotting/ coag. time). Heparin has immediate effect whereas the coumarins take a few days AND the coumarins do affect PT AND PTT or APTT. Hence often with, say Warfarin or Coumadin, the physician will order PTT (or APTT) q6h and PT qd to adjust the dose. Also, from my understanding, the low molecular weight (fractionated) heparins (e.g., Lovenox) and subsequent therapy w/ one of the coumarins are the s.o.p. now (for what that's worth).