Hypodermic syringes and needles are occasionally submitted to this laboratory for analysis for insulin residues. Extraction by simple flushing with 0.9% saline, followed by radioimmunoassay of the washings, has been found to be quite adequate in the cases so far encountered. However, in order to obtain more confidence in this method, and to answer some pertinent questions regarding the variation in the amounts of insulin residue in syringes and needles in different circumstances, the experiments described below were carried out.
Radioiodinated insulin (Radiochemical Centre, Amersham, Bucks) was added to Soluble Insulin B.P. (40 unitslml-Wellcome Ltd., Beckenham, Kent) to give a final activity of 10,000 counts per minute. This solution was taken up into 5ml plastic hypodermic syringes and expelled through 21G (1-5 inches long) or 25G (0.94 inches long) needles. The needles and syringes were separated and the syringes washed out with 2ml of 0.9:/, saline by drawing up and expelling the liquid five times. The radioactivity in the washings was measured in a gamma counter. Needles were either placed in the gamma counter without further treatment or were washed out with 5ml saline and the washings and needle counted separately. Some needles were allowed to fall l m onto a hard surface to determine how much of the liquid contents could be dislodged in this way; others were stuck into a wad of absorbent cotton wool for 15 minutes to determine how much of the contents would be drawn out by capillary action.
The results of these experiments were as follows, (the figures in brackets are the number of items used) :
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The average recovery of insulin residues from freshly used syringes (5) and needles (5) was 95% (range, 87%-101%). I n contrast, the recovery from syringes (4) and needles (4) that had been allowed to dry and then left on the open bench for 1 month, was only 15% (range, 9%-19%).
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The average residue in syringes (8) which had been filled and then emptied as completely as possible was 0-05m1, which represents 2 units of insulin at 40 unitslml strength.
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Syringes (2), which had been filled with insulin solution, were washed out by filling with saline and emptying. Five or six such flushes were required before the residual insulin level dropped below the detection limit of the radioimmunoassay (5 unitslml) .
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Both 21G and 25G needles, when separated from filled syringes, retained approximately 0-02ml of insulin solution, which is equivalent to 0.8 units of insulin at 40 unitslml strength.
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When needles (5), filled with insulin solution, were dropped twice onto a hard surface, on average, 83% (range, 66%-94%) of the liquid content was lost. The minimum insulin residue detected after this treatment was lOOm units.
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No significant loss of insulin occurred when the points of needles (3), filled with insulk solution, were stuck into a wad of absorGent cotton wool. 1t would appear that the liquid remaining in needles is primarily in the Luer fitting and cannot be drawn out through the needle in this way.