How accurate are insulin mixtures prepared by the patient?

The accuracy of mixing two types of insulin in one syringe was studied in 41 insulin-dependent diabetic patients who prepared a mixture of two fluids (saline and sterile water) with either a disposable, 1-ml plastic syringe with a fixed needle, i.e., with a small dead space, or one equipped with a separate needle. Syringes with fixed needles delivered the two components very accurately and were associated with only slight wastage of insulin of 0.011 ml per injection. The syringe with a separate needle delivered 0.033-0.065 ml more than the intended dose of the "first insulin" and correspondingly less of the intended dose of the "second insulin," since the total dose delivered was approximately correct. The magnitude of the insulin wastage of two daily injections was calculated to be 64 ml per patient annually, which could be reduced to 8 ml per patient if fixed needles were used. It is therefore recommended that patients preparing mixtures of two insulins in the same syringe abandon syringes with separate needles.