Falsely Low Serum hCG Level in a Patient With Hydatidiform Mole Caused by the "High-Dose Hook Effect"

Case Presentation A 34-year-old women came to the emergency room complaining of shortness of breath and vaginal bleeding. According to the date of her last menstrual period, she was in her 14th week of pregnancy. She had had 3 previous normal pregnancies and 1 elective abortion. Physical examination revealed a small amount of vaginal bleeding and a 20-week-sized uterus, which was confirmed by an ultrasonographic scan. Other sonographic findings included intrauterine echogenic material and the absence of a fetal heartbeat. Given these findings, the diagnosis of molar pregnancy was made. The patient also had clinical signs consistent with thyrotoxicosis: general thin appearance, mild anxiety, elevated heart rate, elevated respiratory rate, mildly elevated blood pressure, moderate systolic murmur, and increased deep tendon reflexes. The patient was admitted to the hospital for surgical evacuation of the molar pregnancy. On admission, routine laboratory tests of the patient’s blood revealed mild anemia and normal electrolyte levels. Evacuation of the molar pregnancy was performed the second day of hospitalization. One hour after surgery, the first serum human chorionic gonadotropin (hCG) level was 778 mIU/mL (778 IU/L), as measured by the hCG Bayer Immuno 1 immunoassay. When the second serum hCG level was performed 13 hours after surgery, the printout showed no number and was flagged with “>,” indicating that the concentration was greater than the highest standard and that the sample needed to be diluted. After doing serial dilutions of the second sample, the actual hCG level was found to be 819,000 mIU/mL (819,000 IU/L). Recognizing the jump in hCG from 778 mIU/mL to 819,000 mIU/mL to be unusual for 2 consecutive samples drawn 12 hours apart, the technologist suspected the “high-dose hook effect” and performed serial dilutions on the first sample—which revealed an actual hCG level of 1,811,700 mIU/mL (1,811,700 IU/L). On followup, the serum hCG level decreased, displaying a normal regression curve (Table 1). The pathology report of the evacuated tissue was “complete hydatidiform mole.”