Inhibin B plus LH vs GnRH agonist test for distinguishing constitutional delay of growth and puberty from isolated hypogonadotropic hypogonadism in boys.

The distinction between constitutional delay of growth and puberty (CDGP) and isolated hypogonadotropic hypogonadism (IHH) in males with delayed puberty is difficult but important for timely treatment. We assessed the accuracy of the GnRH agonist test (triptorelin 0·1 mg) in comparison with inhibin B alone or in combination with basal LH for the diagnosis of IHH. Ninety-seven prepubertal males aged 13·7-17·5 year, with testicular volumes ≤4 ml, were examined every 6 months. CDGP was defined by a testicular volume ≥8 ml after 18 months, and IHH was defined by a testicular volume <5 ml after 24 months follow-up. Inhibin B concentrations were measured by ELISA, and LH concentrations were measured by CLIA. At follow-up, the cohort comprised 52 boys with CDGP and nine with IHH. The other patients were lost for follow-up (n = 10), had not reached follow-up yet (n = 20) or did not reach a definite testicular volume (n = 6). Basal LH <0·3 IU/l, stimulated LH (4 h) <5·3 IU/l or inhibin B <111 pg/ml had 100% sensitivity for IHH. Only LH (4 h) <5·3 IU/l had a specificity of 100%, and the specificities of basal LH <0·3 IU/l (88%) or inhibin B <110 pg/ml (92%) were lower. The combination of LH <0·3 IU/l with inhibin B <111 pg/ml increased the specificity to 98·1%. The LH response 4 h after GnRH agonist stimulation has an excellent accuracy for the diagnosis of IHH in prepubertal boys with delayed puberty. However, the measurement of inhibin B and basal LH in combination is a valid, reliable and less-invasive alternative test.