Methodology

University Hospital,  East Madison Hospital, Digestive Health Clinic

The venipuncture MUST be atraumatic to avoid hemolysis and contamination with tissue factors and platelets. Tube MUST be allowed to fill up to "fill" line indicated on tube.

If unable to collect 1.8 ml tube (pediatrics only) please call UWHC Coagulation Lab at (608)263-8040 or UWHC Special Coagulation Lab at (608) 263-5005 for alternate tube and instructions.

Centrifuge specimen to yield platelet poor plasma (platelet count should be less than 10K/uL). Separate plasma and transfer to plastic tube or vial. Freeze plasma at -20°C or below.

Transport whole blood specimen in an unopened tube at room temperature to the laboratory within 24 hours of collection. Specimens that have been opened and spun must be less than 4 hours old when received by the laboratory. Otherwise, transport frozen plasma aliquot on dry ice.

Grossly hemolyzed specimens, clotted specimens, partially filled tubes and specimens greater than 24 hours old when received by the laboratory are not acceptable.

INR is the International Normalized Ratio which takes into account the instrumentation and reagent system used at UWHC. The INR is intended to standardize reporting of the prothrombin time. For patients NOT on anticoagulants, the reference range for the INR is 0.9-1.2. The recommended therapeutic range for warfarin is an INR of 2.0-3.0. (Exception: The recommended range for many mechanical valves is INR 2.5-3.5.) Reference: CHEST 2012; 141 pgs e5925-e5945.

Heparin concentrations exceeding 1.2 IU/mL may prolong the INR and thus interfere with monitoring of vitamin K antagonists such as warfarin.

Inhibitors of the Lupus type anticoagulants can influence prothrombin and lead to INR’s that do not accurately reflect the true level of anticoagulation.

Direct thrombin inhibitors (bivalrudin, dabigatran, etc.) in therapeutic dose result in prolonged prothrombin times. 

Daptomycin therapy can cause a false INR elevation in a patient who had been receiving warfarin.

Results may be affected by polycythemia.  A hematocrit greater than 60% creates an incorrect anticoagulant to plasma ratio resulting in a falsely elevated INR result.