LAUSR

OBJECTIVE Little is known about the ratio between luteinizing hormone (LH) and follicle-stimulating hormone (FSH) during infancy. This study aimed to evaluate serum and urinary LH/FSH as a marker of sex with age-specific cutoffs in healthy infants. DESIGN A prospective, longitudinal cohort study of healthy infants aged 0-1.2 years. METHODS In total, 236 healthy infants (122 boys, 114 girls) from The COPENHAGEN Minipuberty Study (ClinicalTrials.gov ID: NCT02784184), with 567 serum and 603 urine samples, were included. Measures of diagnostic accuracy, including sensitivity and specificity, were used to assess the ability of LH/FSH to detect sex in healthy infants. RESULTS In both serum and urine, LH/FSH was highest in males with minimal overlap between the sexes. In contrast to isolated LH and FSH concentrations, LH/FSH ratios in both serum and urine were excellent markers of sex from 0-1.2 years with median sensitivities and specificities ranging from 93-100% with correspondingly narrow 95% confidence intervals. CONCLUSIONS Serum and urinary LH/FSH ratios are excellent discriminators of sex in healthy infants during the entire first year of life. The clinical role and application of the ratio remains to be elucidated.