Objective To determine the accuracy and reproducibility of methods used in practice to measure small dose volumes commonly used in the Paediatric Intensive Care Unit (PICU).
Method Nursing staff were surveyed to determine how they would measure two different volumes of insulin for subsequent dilution in 50 ml of a diluent to produce 2.5 and 5 units in 50 ml. Four methods were identified. In Methods A and B a small excess of volume was drawn into the 1 ml syringe via a filter needle, which was then removed. Method A, the excess was removed leaving the volume required in the syringe then a needle was attached prior to transfer to the 50 ml syringe. Method B, the needle was primed by attaching the needle prior to expelling excess. Methods C and D involved initially diluting 0.5–5 ml, method D different by priming the needle prior to transferring the volume required into the 50 ml syringe. Simulations of each method were repeated 10 times in a pharmaceutics laboratory to determine their accuracy. Weights were used to quantify methods A and B; UV spectroscopy was used to assess the concentration of the final dilution of methods C and D. Results are expressed as mean percentages (±SEM) of simulated insulin doses.
Results For method A, both 2.5 and 5 unit measurements consistently yielded the equivalent of zero units. In method B attempts to measure 2.5 units produced an average of 2.7±0.26 units and 5 units resulted in 5.23±0.5 units. In method C attempts to measure 2.5 units yielded 1.8±0.74 units and 5 units resulted in 3.5±0.98 units. In method D attempts to measure 2.5 units yielded 2.4±0.59 units, and 5 units gave 5.58±0.28 units.
Conclusion The methods described to deliver small volume doses of insulin on PICU provide inaccurate final concentrations. The inability to reproduce accurate concentrations for each dose could have significant clinical effects for the PICU patient, especially for drugs with a narrow therapeutic index for example, insulin.1 2 Dilution methods are generally considered to be more accurate.3However, this small study indicates that for volumes of ≥0.05 ml a direct withdrawal method may be more accurate, but a dilutional method produces a better reproducibility. However the present study must be extended for confirmation. This study also highlights the need for detailed protocols for measuring small volumes to avoid inter-personnel variation.