Montreal General Hospital Montreal, Canada H3G 1A4
To the editor:
In their recent article in JCEM, Luisi et al. (1) found a mean level of allopregnanolone to be about 50 ng/mL in late pregnancy. The method used by Luisi et al. (1), described in a previous publication (2) involves extraction into an organic solvent (diethyl ether) and passage through a C18 Sep-Pak cartridge, followed by RIA using an antibody made and characterized by Purdy et al. (3). No data were provided as to the specificity of this method.
The antibody used was made to an antigen conjugated at C11 and,
although highly sensitive for allopregnanolone, cross-reacts 17% with
progesterone, 50% with 5
-tetrahydoprogesterone and
3
-dihydroprogesterone, and to a minor extent with some other
steroids (4). The original method of Purdy et
al. (3) involved purification of an extract with
high-performance liquid chromatography. However, Luisi et
al. (1) used only C18 Sep-Pak cartridges for
purification.
In our experience, passage through a Sep-Pak cartridge does not remove
progesterone, 5-dihydroprogesterone, or many other steroids. Unless
their Sep-Pak cartridges have properties differing from ours (also
obtained from Waters Corp.), then progesterone, at a concentration of
approximately 200 ng/mL in late pregnancy, would account for about 34
ng/mL of their 50 ng/mL value, while 5
-dihydroprogesterone would
account for an additional 3 ng/mL. Even in nonpregnant patients
(2), competing steroids would be expected to give falsely
high levels. Indeed, the values for nonpregnant women (3)
and our values in nonpregnant women and men, as well as those in
pregnancy (4), are lower than those found by Genazzani
et al. (2).
As has been found previously for cortisol, it is essential to define the specificity of an assay for the particular milieu in which it is used. Thus, an assay validated for plasma cannot be assumed to be accurate for urine (5, 6); nor can an assay validated for plasma in nonpregnant adults be assumed to be satisfactory for that of newborns or pregnant women (7).
Received October 19, 2000.
References