The New WHO

June 9th, 2010 § 12 comments § permalink

A first test of male fertility is the semen analysis.  You do your thing, and a technician counts the sperm, sees how they’re moving, what they look like and whether they’re alive.  For decades, the World Health Organization has published criteria for these numbers to alert a man that he might have a problem when it comes to impregnating a woman.  Until recently, the numbers were a consensus of expert opinion, but in the latest edition, the WHO criteria changed substantially.

What the WHO is currently doing is to dispense with expert opinion, and just lay the numbers out for all to see.  Table II from the paper shows the numbers for men from couples who conceived within a year.  Take sperm concentration, for example.  For centile 5, the sperm concentration is 15 million per ml.  That means that only 5% of couples where the man had 15 million/ml sperm or less conceived within a year.  For centile 50, the concentration was 73 million/ml, meaning that 50% of couples conceived within a year when the sperm concentration was up to that number.  You get the idea.

The problem is that people like cutoffs, and in the latest edition, the WHO chose centile 5 as the line in the sand.  It’s a good number for thinking that below it, couple infertility likely involves the male.  But keep in mind that at centile 10, only 10% of couples conceived within a year.  In other words, having sperm numbers above the centile 5 cutoff doesn’t guarantee that the sperm are trouble free.

Frankly, I think the WHO numbers are most useful to get a ballpark idea of how fertility may be related to what’s inside the semen.  I prefer the approach David Guzick and colleagues took, where they applied a statistical method called Classification and Regression Tree (CART) analysis to sperm, which gives two cutoffs in a “green light, yellow light, red light” fashion.  For example, CART analysis came up with 13.5 million/ml and 48 million/ml for sperm concentration.  At 13.5 million/ml sperm or less, the “red light,” couple infertility likely involves the male.  At 48 million/ml or more, your sperm probably are “green light” good to go.  Between 13.5 million/ml and 48 million/ml, the “yellow light,” sperm may or may not be the problem.  You can find the Guzick CART cutoffs here.

A lot of people, including doctors and fertility specialists, are confused about the new WHO cutoffs.  Expect a little consternation about them for a bit.

WHO Table II Distribution of values, lower reference limits and their 95% CI for semen parameters from fertile men whose partners had a time-to-pregnancy of 12 months or less

N Centiles


2.5 (95% CI) 5 (95% CI) 10 25 50 75 90 95 97.5

Semen volume (ml) 1941 1.2 (1.0–1.3) 1.5 (1.4–1.7) 2 2.7 3.7 4.8 6 6.8 7.6
Sperm concentration (106/ml) 1859 9 (8–11) 15 (12–16) 22 41 73 116 169 213 259
Total number (106/Ejaculate) 1859 23 (18–29) 39 (33–46) 69 142 255 422 647 802 928
Total motility (PR + NP, %)* 1781 34 (33–37) 40 (38–42) 45 53 61 69 75 78 81
Progressive motility (PR, %)* 1780 28 (25–29) 32 (31–34) 39 47 55 62 69 72 75
Normal forms (%) 1851 3 (2.0–3.0) 4 (3.0–4.0) 5.5 9 15 24.5 36 44 48
Vitality (%) 428 53 (48–56) 58 (55–63) 64 72 79 84 88 91 92

*PR, progressive motility (WHO, 1999 grades a + b); NP, non-progressive motility (WHO, 1999 grade c).