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162) A 46-year-old woman in good health was seeing her internist as part of a routine physical examination. Because she reported 6 months of amenorrhea, serum and urine human chorionic gonadotropin (hCG) levels were measured to rule out pregnancy. The serum hCG level was 10 IU/L (reference range, 5 to 20 IU/L), and a positive urine hCG result was obtained. Serum hCG was measured in another sample 7 days later and was 12 IU/L. When hCG was measured with a different immunochemical method, the serum hCG result was 11 IU/L. The patient’s serum follicle-stimulating hormone (FSH) level was 130 IU/L (reference range, 45 or higher IU/L). Which one of the following is the most probable cause of the detectable hCG level in this patient?

Positive interference from human anti-animal antibodies. Normal intrauterine pregnancy. Ectopic pregnancy. Synthesis and release of hCG from the pituitary gland. Presence of a hydatidiform mole.

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• Human chorionic gonadotropin (hCG) was first identified in the pituitary gland in 1976. Pituitary hCG has the same amino acid structure as the intact hCG that is secreted during pregnancy. Pituitary hCG is produced at low levels during the menstrual cycle and is a sulfated variant of hCG.

• hCG levels increase with increasing age. In perimenopausal and postmenopausal women, ovarian steroid synthesis decreases. As a result, the negative feedback control on gonadotropin-releasing hormone (GnRH) decreases, and the GnRH pulse significantly increases. GnRH stimulates the release of pituitary hCG and could be the cause of the low levels of hCG seen in some perimenopausal and postmenopausal women.

• Pituitary hCG can be excluded in perimenopausal and postmenopausal women as the source of the low hCG level by treating the patient with estrogen-progesterone hormone replacement therapy. If the source of the hCG is the pituitary, then the hCG level will be suppressed.

• Follicle-stimulating hormone (FSH) levels can also be used to investigate the source of the hCG. FSH levels are suppressed during pregnancy and increased in perimenopausal and postmenopausal women. In perimenopausal women (41 to 55 years old), the FSH reference range is 45 IU/L or greater. In pregnancy, the FSH level is significantly lower. If a low hCG level is obtained in perimenopausal women, reflex FSH testing should be performed. The FSH of 130 IU/L in this patient suggests that the patient is not pregnant and that the pituitary gland is probably the source of the low hCG level.

• Human anti-animal antibodies (HAAAs) have a molecular mass of more than 120 kDa and are not excreted into the urine. Therefore, a positive urine hCG result, together with obtaining essentially the same serum hCG result using two different immunochemical methods with a different set of antibodies to measure hCG, suggests that HAAAs are not the cause of the low hCG level that was obtained in this patient.

Snyder JA, Shannon H, Parvin CA, et al: Diagnostic considerations in the measurement of human chorionic gonadotropin in aging women. Clin Chem 2005;51(10):1830–1835.

Cole LA: Biological functions of HCG and HCG-related molecules. Reprod Biol Endocrinol 2010;8:(102)1–14.

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Clinical Pathology: General Principles, Lab Management, Clinical Chemistry

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