COMPARISON OF THE PRECISION OF MEASUREMENTS IN THREE TYPES OF MICROPIPETTES ACCORDING TO NCCLS EP5-A2 AND ISO 8655-6
Micropipettes or piston pipettes are used to make most volume measurements in fields such as health, chemistry, biology, pharmacy and genetics. Laboratories must ensure that results obtained using these instruments are reliable; therefore, it is necessary to calibrate micropipettes. Before the start of the calibration process, we must check the precision of measurements. The objective of this work is to compare several methods for calculating the precision of three kinds of micropipettes according to the reference value in ISO 8655-6. The medical tests will not have accurate results, if the volume of the liquid doesn’t transfer precisely by micropipettes. Thus, the physician might potentially face problems in the disease diagnosis and its control. In the NCCLS EP5-A2, there is a method to specify and assess the precision of micropipettes by using CV (Coefficient of Variation). Also there are other methods to estimate and test the CV theory, in the formal statistics texts which could be applied to assess the micropipettes precision. In this research we evaluate the precision of lab micropipettes. Three brands of micropipettes, A, B and C are assigned to measure the distilled water mass by using accurate scale which is accurate up to 10-6 to measure 50-gram weights. The experimental environment is a metrology lab which is approved by Iran Standard and Industrial Researches Organization. A technician sampled at the beginning of the experiment and then after 2 hours, the same technician repeated the sampling. Overall, each micropipette is used to measure 40 times with 10-repeat times for single measurement in 28 work days. Common statistical methods are used to estimate and test the CV. Point estimation of CV for micropipettes A, B and C were 0.50%, 0.64% and 1.56%, respectively. Furthermore, the upper limit of 95% confidence bounds for these three micropipettes using the exact method were 0.53%, 0.69% and 1.65%, respectively. Micropipette A met the ISO 8655-6 standard level, but micropipettes B and C did not. On average, measurement errors in micropipettes B and C were respectively 30% and 3.11 times more than micropipette A. By using the approach of CLS EP5-A2 and confidence interval for CV, precision of the three micropipettes were compared. Only one of them met the ISO 8655-6 standard level, but the others failed.