ESPE Abstracts

Background: False-negative results can occur an extremely high level of substrate at the assay system. This is called the ‘high-dose hook effect’.

Case presentation: 14 year-old female patient was referred with short stature, amenorea and hirsutism. Height, weight and blood pressure were 140.5 cm (SDS:−3.5), 43.4 kg (SDS:−1.6) and 120/80 mmHg. She had stage 1 Tanner breast, stage 5 pubic hair and hirsutism (mFG scale: 28) with android distribution. The clitoris was enlarged 4 cm in longitudinal axis and 1.5 cm in transverse diameter of the glands. Laboratory: Sodium 138 mmol/L, potasium 4.3 mmol/L, 17-OHP <0.04 ng/ml (0.2–1.3), DHEA 720.6 ug/dL (35–430), androstenedione 13 ng/mL (0.3–3.3), testosterone 4,4 ng/ml (0.1–0.7), ACTH 319 pg/mL (4.7–48.8), cortisol 7.8 ug/dL (6.7–22.6), renin 69.7 mIU/mL (2.8–39), aldosterone 664.1 pg/mL (25–315), LH 3.1 mIU/ml, FSH 6.3 mIU/ml, estradiol <20 pg/mL, progesterone 31.9 ng/ml (25–-315), prolactin 12.6 ng/ml. Pelvic ultrasonography was normal (uterus 8 ml and ovaries 2/2.3 ml). The karyotype was 46, XX. Unexpected result of low serum 17-OHP level led to the suspicion of a ‘high-dose hook effect’. The measurement was repeated after 1/10 dilution of serum, and high level of 17-OHP was detected (115.4 ng/ml) with ELISA test (DiaMetra, Segrate, Italy). Homozygous p.I173N (c.518T>A) mutation was detected in the CYP21A2 gene. These values confirmed the diagnosis of 21-hydroxylase deficiency and hydrocortisone therapy was iniated.

Discussion: Excessive accumulation of steroid precursors are occur due to the shunt throught the adrenal androgen biosentetic pathway. Large quantities of antigen impair antigen-antibody binding, resulting in low antigen levels (false negative result) in laboratory assays which can be satisfied by dilution of the sample or a change of sample antigen to antibody ratio either by assay reformulation.

Conclusion: This is the first case of the hook effect for 17-OHP immunoassay in 21-hydroxylase deficiency. Hook effect needs to be suspected in patients with CAH when steroid precursors are incompatible.