Approach Considerations
Blood levels of warfarin are neither readily available nor helpful. Specific levels of superwarfarin rodenticides (eg, brodifacoum) may be useful in cases in which the ingestion is denied or for purposes of estimating the necessary duration of vitamin K1 therapy. However, most reference laboratories do not perform this analysis. (National Medical Laboratory [NMS] in Willow Grove, Pa, offers a qualitative screen for anticoagulant rodenticides and a quantitative analysis for brodifacoum.)
The anticoagulant effect is best quantified by baseline and daily repeated measurement of the prothrombin time (PT) and the International Normalized Ratio (INR), which may not be elevated until 1-2 days postingestion. [10] A normal PT 48-72 hours after ingestion rules out significant ingestion.
Blood levels of vitamin K–dependent clotting factors (II, VII, IX, and X) are decreased if measured, but these are rarely available in a timely fashion and usually do not aid in clinical management. However, depressed levels may provide supporting evidence for suspected poisoning by warfarin or superwarfarins. [13]
Note that a "mixing study" using an aliquot of normal serum mixed with the patient's serum will restore an abnormal PT to normal. This may be useful when trying to distinguish anticoagulation caused by warfarin or a superwarfarin from that caused by a factor inhibitor (eg, lupus anticoagulant) or antifactor antibodies. [14]
Other laboratory tests that may be indicated include a blood count for baseline hemoglobin and/or hematocrit to assess for anemia if the ingestion is more remote. A blood type and crossmatch or antibody screening is indicated if substantial blood loss is suggested.
In addition, other laboratory tests (eg, acetaminophen level) or toxicology screening may be indicated to rule out co-ingestions. If intracranial bleeding is suspected, obtain a noncontrast computed tomography (CT) scan of the head.
