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Letters to the Editor |
1 Department of Laboratory Medicine, Kaohsiung Medical, University Hospital, Kaohsiung, Taiwan
2 Department of Obstetrics, and Gynecology, Faculty of Medicine, College of Medicine, and
3 Department of, Clinical Biochemistry, Faculty of Biomedical Laboratory Science, College of Health Sciences, Kaohsiung Medical University, Kaohsiung, Taiwan
aAddress correspondence to this author at: Department of Clinical Biochemistry, Faculty of Biomedical Laboratory Science, College of Health Sciences, Kaohsiung Medical University Hospital, 100 Shih-Chuan 1st Rd., Kaohsiung, Taiwan. Fax 886-7-238-2669; e-mail tsliyu{at}kmu.edu.tw.
To the Editor:
A 16-year-old girl (gravida 0, para 0) presented to the emergency department with a 2-week history of nausea, vomiting, vaginal spotting, and lower leg edema. On examination we found a palpable lower abdominal mass. The patient acknowledged recent sexual activity but denied having any sexually transmitted diseases.
Molar pregnancy was suspected because of the typical "snowstorm" appearance observed with ultrasound examination. The quantitative serum ß-subunit of human chorionic gonadotropin (ß-hCG) concentration was 746.20 IU/L (reference interval, <0.5 to 2.90 IU/L; reportable interval, 0.5–1000 IU/L). The result of a qualitative urine hCG test, however, was negative.
Human error was initially suspected as the cause of the negative result in the first qualitative urine hCG test. Two experienced and well-trained senior medical technologists repeated the urine hCG test twice, but the results were still negative. The patient’s nurse also indicated that she did not think there had been mislabeling of the urine sample. Because of this discrepancy, we had a discussion with the clinicians and concluded that a hook effect might be responsible for the negative urinary hCG result and for the relatively low serum ß-hCG concentration. We therefore repeated the tests at a 1 to 10 000 dilution and obtained values for serum ß-hCG and urinary hCG of 3 835 000 IU/L and 4 873 400 IU/L, respectively.
According to the manufacturer’s information of the Beckman Access 2 analyzer, a hook effect will occur when the serum ß-hCG concentration exceeds 1 000 000 IU/L. The manufacturer’s information for the ACON urine hCG One Step Pregnancy Device (Format: FHC-102) does not provide any information about a hook effect, but hook effects occurring in a qualitative urine and a quantitative serum ß-hCG assay have both been reported recently (1)(2).
In this case, we report a novel observation of hCG concentrations high enough to cause a hook effect in both qualitative and quantitative hCG assays. We diluted the urine sample to various concentrations (1:2, 1:4, 1:6, 1:8, and 1:10 dilutions) and performed qualitative analysis of urinary hCG on each (Table 1
). The results showed no hook effect in the qualitative urine hCG test at 
487 000 IU/L, but a hook effect was evident at concentrations of 609 000 to 2 000 000 IU/L. There was complete signal elimination at a concentration of 4 000 000 IU/L.
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According to the Beckman package insert, the hook effect occurs at concentrations >1 000 000 IU/L. We found no hook effect in the quantitative serum ß-hCG at a concentration of 958 750 IU/L, but did observe a hook effect at concentrations of >1 917 500 IU/L to 3 835 000 IU/L (Table 1
). On the basis of our findings, we suggest that, when hydatidiform mole is suspected, the urine sample should be diluted at least 1:10, particularly when the ACON urine hCG One Step Pregnancy Device (Format: FHC-102) is used, to avoid inaccurate urine hCG results.
Hydatidiform mole results from the overproduction by tissue that would typically develop into the placenta. The incidence of molar pregnancy demonstrates marked geographic and ethnic differences, ranging from a highest incidence of 1 in 120–400 pregnancies in Asian countries such as Taiwan, the Philippines, and Japan, to a lowest incidence of 1 in 1000–2000 pregnancies in Europe and the United States (3). This disorder is more common in young women and women nearing the end of their reproductive years (4). In addition, there is a 1% risk of having molar disease in a subsequent pregnancy after a molar pregnancy (5). The classic clinical presentation includes vaginal bleeding with abnormal growth in the size of the uterus. The diagnosis is typically established by markedly increased concentrations of ß-hCG and the characteristic snowstorm appearance on ultrasound.
In conclusion, although modern assay methods have eliminated many laboratory errors, a high-dose hook effect might occur in urine hCG tests for hydatidiform mole. To avoid false-negative hCG results, appropriate sample dilution must be performed. Furthermore, the details of the hook effect should be clearly explained in the manufacturer’s instructions. Medical technologists and physicians must also be aware of the hook effect to accurately interpret urinary hCG and serum ß-hCG test results. Our report identifies the concentrations at which the hook effect occurred for both of these hCG methods for the lot numbers tested. This knowledge may be useful to clinicians and clinical laboratory specialists to avoid delays in diagnosis and therapy.
Footnotes
1 These authors contributed equally to this work. 
References
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