title: Uncertainty associated with assessing semen volume: are volumetric and gravimetric methods that different?
abstract: The World Health Organization laboratory manual for the examination of human semen suggests that an indirect measurement of semen volume by weighing (gravimetric method) is more accurate than a direct measure using a serological pipette. A series of experiments were performed to determine the level of discrepancy between the two methods using pipettes and a balance which had been calibrated to a traceable standard. The median weights of 1.0ml and 5.0ml of semen were 1.03 g (range 1.02-1.05 g) and 5.11 g (range 4.95-5.16 g), respectively, suggesting a density for semen between 1.03g and 1.04 g/ml. When the containers were re-weighed after the removal of 5.0 ml semen using a serological pipette, the mean residual loss was 0.12 ml (120 μl) or 0.12 g (median 100 μl, range 70-300 μl). Direct comparison of the volumetric and gravimetric methods in a total of 40 samples showed a mean difference of 0.25ml (median 0.32 ± 0.67ml) representing an error of 8.5%. Residual semen left in the container by weight was on average 0.11 g (median 0.10 g, range 0.05-0.19 g). Assuming a density of 1 g/ml then the average error between volumetric and gravimetric methods was approximately 8% (p < 0.001). If, however, the WHO value for density is assumed (1.04 g/ml) then the difference is reduced to 4.2%. At least 2.4-3.5% of this difference is also explained by the residual semen remaining in the container. This study suggests that by assuming the density of semen as 1 g/ml, there is significant uncertainty associated with the average gravimetric measurement of semen volume. Laboratories may therefore prefer to provide in-house quality assurance data in order to be satisfied that 'estimating' semen volume is 'fit for purpose' as opposed to assuming a lower uncertainty associated with the WHO recommended method.
authors.affiliation: a Fertility Unit, Nottingham University Hospital , Nottingham , UK.
authors.forename: Bryan
authors.initials: B
authors.lastname: Woodward
authors.affiliation: a Fertility Unit, Nottingham University Hospital , Nottingham , UK.
authors.forename: Nicole
authors.initials: N
authors.lastname: Gossen
authors.affiliation: a Fertility Unit, Nottingham University Hospital , Nottingham , UK.
authors.forename: Jessica
authors.initials: J
authors.lastname: Meadows
authors.affiliation: a Fertility Unit, Nottingham University Hospital , Nottingham , UK.
authors.forename: Mathew
authors.initials: M
authors.lastname: Tomlinson
mesh_terms: D006801:Humans; D007753:Laboratories; D008297:Male; D012661:Semen; D055101:Semen Analysis / Q000379:methods*; D035501:Uncertainty
keywords: Uncertainty; WHO guidelines; semen volume