Stability of diluted heparin sodium stored in plastic syringes

The stability of heparin diluted in 0.9% sodium chloride injection and stored in plastic syringes for a three-week period was studied. Heparin activity was assayed by the activated partial thromboplastin time (APTT) method. Heparin sodium (25,000 units/ml) was diluted to 500 units/ml and stored in 50-ml polypropylene syringes. Concentrations were compared in two brands of syringes stored at room temperature in the dark. In another experiment controlled for order-related assay errors, heparin was stored in one brand by syringe at either 0-4 degrees C or room temperature. There was a statistically decrease in heparin activity over three weeks in both syringes and at both 0-4 degrees C and room temperature. However, the overall drop in activity was only about 8%. Analysis of covariance confirmed significant regression with time at both temperatures. An unexpected finding was that heparin at 500 units/ml consistently assayed higher than this value. A study of the effect of glass and plastic showed that when heparin was diluted into either a glass or plastic container, there was significantly less heparin activity in the glass containers within two hours. One possible explantation for this phenomenon is absorption of heparin to glass surfaces. It was concluded that heparin can be stored in polypropylene syringe for up to three weeks without refrigeration. However, once diluted, heparin should not be stored in glass containers.