The orchidometer was introduced in 1966 by Swiss pediatric endocrinologist Andrea Prader of the University of Zurich. It consists of a string of twelve numbered wooden or plastic beads of increasing size from about 1 to 25 milliliters. Doctors sometimes informally refer to them as "Prader's balls", "the medical worry beads", or the "endocrine rosary”.
The beads are compared with the testicles of the patient, and the volume is read off the bead which matches most closely in size. Prepubertal sizes are 1–3 ml (yellow), pubertal sizes are considered 4 ml and up to 12 ml (orange) and adult sizes are 15–25 ml (red).
Discrepancy of testicular size with other parameters of maturation can be an important clue to various diseases. Small testes can indicate either primary or secondary hypogonadism. Testicular size can help distinguish between different types of precocious puberty. Since testicular growth is typically the first physical sign of true puberty, one of the most common uses is as confirmation that puberty is beginning in a boy with delay. Large testes (macroorchidism) can be a clue to one of the most common causes of inherited generalised learning disability, fragile X syndrome.
Stephen Shalet, a leading endocrinologist is reported to have told, "Every endocrinologist should have an orchidometer. It's his stethoscope."