Hi Dominic, some years ago I used an R&D kit with detection limit at 21 pg/ml and if I remember correctly most of my SD male, approx 3 month old controls were below detection limit (in undiluted serum). I was pretty dissapointed of wasting money then and I think I inputted the value as 0 (got wonderfull statistics...). Anyhow, VERY VERY low.