Estimating the intra- and inter-individual imprecision of manual pipetting

Background: Despite the importance of manual pipetting of fluids such as water, solutions, buffers, reagents, or biological samples in daily laboratory practice, the intra-and inter-individual imprecision of this activity has not been recently described in scientific publications.Methods: Twenty laboratory operators were randomly enrolled for this study. Imprecision of manual pipetting was estimated by asking each laboratory professional to dispense 1 mL, 100 mu L or 10 mu L of distilled water for 10 consecutive times with three certified pipettes into a 50-mL plastic container placed into a gravimetric balance. The weight of the water dispensed was systematically recorded for each of the 10 repeated attempts, and the inter-and intra-operator imprecision was finally calculated and expressed as coefficient of variation (CV%).Results: The mean intra-individual imprecision was 5.7% (range, 0%-11.8%) for pipetting 10 mu L, 0.8% (range, 0.4%-1.9%) for pipetting 100 mu L, and 0.2% (range, 0.1%-0.5%) for pipetting 1 mL. Overall, the mean inter-individual imprecision was 8.1% for pipetting 10 mu L, 1.1% for pipetting 100 mu L and 0.4% for pipetting 1 mL. A significantly inverse correlation was found between intra-individual pipetting imprecision and the amount of water dispensed (r = -0.80; p < 0.001). No significant correlation was observed between individual pipetting performance and sex, age, qualification, and years of experience in the laboratory.Conclusions: The results of this study show that manual pipetting is plagued by a considerable intra-and inter-individual imprecision, which is inversely correlated with the amount of fluid dispensed.