Reliability of an intravenous intermittent access port (saline lock) for obtaining blood samples for coagulation studies

BACKGROUND: Patients admitted to rule out myocardial infarction often receive a heparin infusion that requires measurement of partial thromboplastin time every 6 to 8 hours until coagulation values stabilize. If the patient's acuity at admission does not necessitate placement of arterial or central catheters, insertion of an intermittent intravenous access port (saline lock) would decrease the need for frequent venipuncture. Many studies on obtaining blood samples via arterial and central catheters have been reported, but no reports have described the method of using a saline lock or the amount of blood discarded during that procedure. OBJECTIVE: To evaluate the efficacy of inserting a saline lock specifically for obtaining blood samples and to determine the amount of blood that must be discarded in order to obtain prothrombin and partial thromboplastin times that match the values from blood samples obtained via venipuncture. METHODS: Coagulation values determined with 2 consecutive blood samples obtained via the saline lock after the first 0.5 mL of blood withdrawn was discarded were compared with measurements determined with a blood sample obtained via venipuncture. RESULTS: Prothrombin and partial thromboplastin times were not significantly different among the 3 blood samples. CONCLUSION: The untoward effects of frequent venipunctures to obtain blood samples for measurement of prothrombin and partial thromboplastin times can be lessened by using an 18-gauge saline lock to obtain blood samples and discarding the first 0.5 mL of blood withdrawn before the samples for coagulation studies are collected.