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Measurement uncertainty and sperm motility: quantification, estimation or guess-timation?

It seems logical to many that the importance of sperm motility is not just a question of ‘how many sperm swim forward?’ but ‘how fast do they swim?’ and the published evidence on the whole certainly supports this. Ever since the introduction of CASA (computer assisted sperm analysis) in the late 1980s/early 1990s, scores of publications have demonstrated the clinical significance of sperm swimming speed (velocity) in all its guises, in relation to the chance of pregnancy or live birth (see Tomlinson and Naeem, 2018). This makes it all the more surprising that the majority of andrology/ART laboratories adopt manual methods that are unable to provide any kind of quantitative assessment of motility or velocity at all.

Adapted from Tomlinson and Naeem (2018)

Three or four grades of motility?

For those adopting WHO guidance for the past 10 years the debate over whether we choose 4 grades (fast progressive, medium progressive, non-progressive and static) or 3 grades (progressive, non-progressive and static) has now run its course. The basis for ‘losing a grade’ was not scientific but a pragmatic one, with the acceptance of the inherent subjectivity of the assessment and a need to achieve results between technicians which are more reliable and reproducible. The latest WHO guideline reverted back to the guidance published in the late 1980s/early 90s without substantial changes in the evidence-base, presumably because the decision was made that simple progression alone could be misleading and gives no discrimination of sperm swimming speed and as such reduces its clinical value. However, this presents yet another challenge: How can we achieve parity between technicians or even laboratories for the assessment of 4 grades of motility? The evidence from EQA data prior to 2010 was not encouraging – see figure below. Bearing in mind that the grading of sperm motility not only underpins the investigation of the infertile male but is also used as an ‘end-point’ measurement for performance testing of a sperm preparation media, or perhaps a new cryopreservation method or even routine toxicity testing of laboratory consumables, we should be concerned.

Spread of results from the UK National external quality assessment scheme for motility, showing enormous disparity between laboratories for the same assessment of grades a (rapid) and b (sluggish) motility by over 250 centres

Manual estimates - continued uncertainty

More importantly, manual motility is performed down the microscope usually over several minutes with no chance of significantly restricting the duration to a ‘moment in time’ , which is always likely to lead to over-estimation of the proportion of progressive sperm. The reason for this is logical, in that although it is simple to assess those cells that are static or non-progressive (going nowhere), the longer the analysis takes, the more time there is for progressive sperm to leave and enter the field of view.

How can uncertainty be reduced?

Performing manual motility from a video, limited to no more than a couple of seconds, and providing more of a ‘snapshot’ or ‘moment in time’ analysis could represent an improvement on the traditional ‘real-time’ approach but doesn’t really help with the level of subjectivity and categorisation according to swimming speed. A more measured approach is to integrate CASA into routine laboratory practice.

Time-limited video loop from SAMi (CASA)

The advantages of this are clear. Not only will this provide reliable ‘time-limited’ estimates of sperm velocity and motility but the additional benefits: removing human error and subjectivity; consistency across all operators; simultaneous assessment of sperm number/motility; reproducibility across ‘sister’ clinics and hard copy data with video clips saved for each assessment

Only ‘quantitative’ motility as opposed to what can only be described as the ‘rough estimate’ afforded by standard manual analyses, can provide the level of reliability needed for ‘end-point measurement’ for use in performance measurement in a number of key areas:

§ Toxicity testing of consumables or other products such as lubricant gels

§ Performance testing of media (cryoprotectant, sperm wash buffer or density gradient)

§ Assessment of pre- and post- preparation sperm quality for ART

§ Testing of procedure performance e.g. sperm preparation or cryopreservation

Just as importantly, keep in mind that alongside 'sperm concentration' (see previous blog) and a highly subjective morphology analysis, assessment of sperm motility forms the bulk of the investigations into the sub fertile male.


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