Causes of Abnormal Screening Results
Causes of Abnormal Screening Results | Specimen Collection and Transport Issues | Medication | Pathologic Conditions | Factor Deficiencies | Factor Inhibitors
Specimen Collection and Transport Issues
Traumatic Collection. Needles larger than 23 gauge should be used for phlebotomy to avoid mechanical disruption of platelets and red blood cells. The venipuncture should be clean (without trauma) to avoid hemolysis and contamination of the specimen with procoagulant factors. Prolonged application of the tourniquet (more than one minute) should be avoided because it can reduce venous circulation and result in production of coagulation factors that can affect test results.
Collection in Glass Tubes. Collection containers must be composed of nonreactive or nonwettable materials such as siliconized glass or plastic. Tubes made of borosilicate, soda lime, or soft glass cannot be used, as these materials may activate the contact pathway of coagulation.
Wrong Citrate Concentration. LabCorp provides blue-stopper collection tubes containing 3.2% (0.109M) buffered sodium citrate for coagulation sample collection. Reference intervals have been established with these tubes. Tubes with alternate citrate concentrations should not be used.7
Inadequate Tube Filling. Evacuated collection tubes must be filled to completion to ensure the proper blood to anticoagulant ratio is achieved.8 Sample tubes less than 90% filled will be rejected.
Heparin Contamination. Heparin contamination can occur when the sample is drawn inappropriately from arterial or central venous line. Blood for coagulation testing should never be collected from heparinized lines. When blood is drawn through an indwelling catheter, the line should be flushed with 5 mL of saline and the first 5 mL of blood discarded. Despite efforts to flush heparin from these lines, heparin contamination can still occur. When heparin contamination is suspected, a thrombin time can be performed (015230 Thrombin Time). A heparin level that will cause only a slight prolongation of the aPTT will cause a thrombin time >150 seconds.
Other Contamination. When multiple tubes are collected, collect sterile tubes and nonadditive [red-stopper] tubes prior to citrate [blue-stopper] tubes. Any tube containing an anticoagulant (ie, EDTA [lavender-stopper], heparin [green-stopper] or oxalate [gray-stopper]) should be collected after the blue-stopper tube is collected. Serum-separator tubes and serum tubes with clot initiators should also be collected after the citrate tubes.
Inadequate Mixing. It is critical that the collected sample be mixed immediately by gentle inversion at least six times to insure adequate mixing of the anticoagulant with the sample. Inadequate mixing can lead to clot formation, depleting coagulation factors and resulting in erroneous results.
Platelet Contamination. Clotting tests for LA are especially sensitive to contamination with platelet components.9 Specimens destined for LA testing should be collected by a method designed for the production of platelet-poor plasma (ie, platelet count is less than 10,000/mL). Double centrifugation of the plasma should be performed.17
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