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NOT EVEN A LITTLE BIT PREGNANT – CA COLON AS CAUSE OF FALSE POSITIVE PREGNANCY TEST

Dayanath, B1; Simpson, I1; Lu, Z X1,2; Doery, J C G1,3

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Pathology - Journal of the RCPA: 2009 - Volume 41 - Issue - p 82
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Introduction: Pregnancy testing is usually performed in women of reproductive age prior to any procedure which could potentially harm a developing embryo. Occasionally, false positive pregnancy tests may occur and have the potential to cause not only delay in treatment but also great consternation for the patient concerned.

Patient: A 38‐year‐old female with advanced colorectal carcinoma (Duke's stage 4) had serum beta human chorionic gonadotropin (hCG) tested prior to receiving chemotherapy. hCG was 74 IU/L which was interpreted as unambiguously positive. Chemotherapy was therefore delayed in spite of the patient believing she could not be pregnant.

Investigations: The luteinising hormone (LH) and follicle stimulating hormone (FSH) were normal for a woman of reproductive age. The initial result was confirmed on two alternate analysers and repeated 1 week later when it had risen to 84. Elimination of heterophile antibodies by use of Scantibodies and urine hCG confirmed the presence of free hCG. Performance of immunohistochemical staining of the original tumour collected 31 months earlier for hCG revealed patchy production of hCG in about 5% of the tumour population. This was seen predominantly at the advancing margin of the tumour.

Discussion: An hCG of 74 IU/L is normally considered unambiguously positive and the absence of heterophile antibodies or peri/post‐menopausal source of pituitary hCG appeared to further increase the possibility of a normal or ectopic pregnancy. However the failure of the hCG to rise rapidly, a normal abdominal ultrasound and the protestations of the patient made further investigation necessary. The demonstration of hCG producing tumour tissue confirmed the likely origin of the circulating hCG.

Conclusion: Laboratories must be constantly aware of the common sources of false positive pregnancy tests including heterophile antibodies, ‘macro’ hCG, pituitary hCG in peri/post menopausal women and tumour tissue itself. The production of hCG by germ cell tumours and teratomas is well recognised but the production by gastrointestinal cancers is much less well recognised. Recent reports suggest that non‐trophoblastic tumours producing hCG are associated with a poor prognosis but a technique to stimulate anti‐hCG antibodies may provide therapeutic promise.

© 2009 Royal College of Pathologists of Australasia