A human chorionic gonadotropin (hCG) test is commonly ordered by gynecologists prior to surgical procedures, in the workup of bleeding abnormalities, and in the follow-up of ectopic and molar pregnancies, to name a few indications. In doing so, occasionally clinicians will find themselves in the diagnostic dilemma of discovering an inexplicable low-level elevation in hCG, such as in a postmenopausal patient. This clinical picture can be confusing and can be concerning for conditions such as postmolar gestational trophoblastic neoplasia (GTN). However, there can be benign causes of this phenomenon.1 To prevent unnecessary worry, investigation of treatments is important. In fact, misdiagnosis and inappropriate treatment of benign, low-level hCG levels with unnecessary chemotherapy is problematic mismanagement of gestational trophoblastic disease (GTD), and a major cause of litigation.
Dr. Emma C. Rossi
Human chorionic gonadotropin is a glycoprotein hormone with two subunits (alpha and beta). It can come from multiple sources, including trophoblastic cells, malignant trophoblastic cells, the pituitary gland, and exogenous sources.1 Its alpha-subunit is identical to that of follicle stimulating hormone (FSH), luteinizing hormone (LH), and thyroid-stimulating hormone (TSH). Its beta-subunit is unique, though very similar to that of LH. The free hCG beta subunit can be produced by nontrophoblastic neoplasms. The gene for the beta subunit of hCG is in close proximity to the beta subunit of LH and increases in gonadotropin-releasing hormone (GnRH) in menopause can result in the stimulation of both genes. Understanding the sources of hCG-like glycoproteins and mechanisms for testing is important when considering possible causes for falsely elevated hCG.
Most commercially available serum hCG assays detect normal intact hCG and free beta subunits. They are typically sandwich assays utilizing antibody binding sites in which a solid-phase anti-hCG antibody to a specific hCG target is then mixed with the patient’s serum, trapping or binding the hCG, which is then treated with an indicator antibody. After being washed with the indicator or “capture” antibody, its relative (quantitative) levels can be measured.1
Urine hCG testing (such as urine pregnancy tests) work through capillary action, drawing the patient’s urine across absorbent pads before reaching a pad which contains anti-hCG antibodies (the detection zone) in the test line. These tests are less sensitive than serum tests, but many can detect hCG levels <15-20 mIU/mL.1
When ob.gyns. are asked to consult on or evaluate persistently low-level elevations of hCG in nonpregnant patients they should consider both malignant and nonmalignant etiologies. Malignant causes include GTN or quiescent GTD (e.g., after treatment of a molar pregnancy or GTN), choriocarcinoma (e.g., ovarian germ cell tumors), and nonchoriocarcinoma malignancies (such as cervical, pancreatic, breast, renal). Nonmalignant causes of hCG elevations in nonpregnant patients include pituitary hCG (in postmenopausal patients), exogenous hCG, and phantom hCG.
The first step in diagnostic workup is to perform a urine pregnancy test. Provided that the serum hCG level is > 20 mIU/mL, the urine HCG should be positive unless the cause of elevated levels is “phantom hCG” from heterophilic antibodies. When patients are exposed to animal antigens (such as in vaccines) they can develop antibodies such as human anti-mouse antibody. These antibodies have affinity to the binding antibodies used in many hCG sandwich assays and form a linkage between the solid phase antibody and the detection antibody creating a false-positive result. This false-positive test is only present in serum testing but not urine tests because the patient’s heterophilic antibodies are not excreted by the kidney and thus not available to create a false-positive result. An alternative method to make the diagnosis of phantom hCG is to request that the hCG testing be run at a different lab with a different assay (which may not react with the same affinity to the patient’s anti-animal heterophile antibodies), or to request that the lab perform serial dilutions. If phantom hCG from heterophile antibodies is at play, serial dilutions will result in a nonlinear dilution response.
If the patient’s urine hCG test is positive, then pregnancy should be ruled out with a transvaginal ultrasound. If negative, an ectopic pregnancy should still be considered (unless not medically plausible, such as in postmenopausal women or women who have undergone hysterectomy). In the absence of an intrauterine or ectopic pregnancy, a positive serum and urine pregnancy test could be from exogenous hCG, from malignancy or pituitary hCG. Use of exogenous hCG can be ruled out by taking a thorough history, with particular focus on asking about weight loss medications and muscle building therapies.
If pregnancy and exogenous hCG are ruled out, clinicians should assess for an occult hCG-secreting malignancy. The lab should be asked to measure the proportion of the free beta subunit of hCG, as this is typically what is secreted by malignancies. CT imaging of the chest, abdomen, and pelvis to search for an occult primary tumor should take place. If the patient has been recently treated for molar pregnancy or GTN, and serum hCG levels reside between 100 and 300 mIU/mL, quiescent GTD should be considered the diagnosis. Determination of the proportion of hyperglycosylated hCG to total hCG can help differentiate active choriocarcinoma from quiescent GTD. After restaging imaging has been done to confirm no measurable metastatic foci, observation can follow with monthly hCG measurements. The majority of these cases will eventually resolve without intervention within a year. Quiescent GTD and persistent low-level HCG in the absence of measurable GTN on imaging or symptoms does not require treatment with chemotherapy or hysterectomy, particularly in women who desire future fertility.2
Once occult malignancy has been ruled out, the remaining potential source of hCG is the pituitary gland. As mentioned earlier, hCG shares its morphology with TSH, LH, and FSH. This can result in cross reactivity and false positives. In the menopausal state, GnRH levels increase and thus so do pituitary LH and hCG levels. To confirm that the pituitary is the source of the low-level hCG levels, the provider should prescribe a course of hormonal treatment such as an oral contraceptive pill for a 2- to 3-month period. This should result in suppression of pituitary hCG, and serum hCG levels, as part of a negative feedback loop. Pituitary source of hCG is a benign condition, and, like quiescent GTD, phantom hCG or exogenous hCG does not require intervention.
Getting to the bottom of persistent low-level hCG elevations can be challenging. By following the step-wise algorithm listed here, clinicians can sequentially test for urine hCG, heterophilic antibodies, elevated free beta-subunit, occult malignancy, and pituitary hCG.
Dr. Rossi is assistant professor in the division of gynecologic oncology at the University of North Carolina at Chapel Hill. She has no conflicts of interest. Email her at obnews@mdedge.com.
References
1. Oyatogun O et al. Ther Adv Reprod Health 2021 Jun 13. doi: 10.1177/2F26334941211016412.
2. Soper JT. Obstet Gynecol. 2021 Feb 1;137(2):355-70.