Doctors are misdiagnosing a form of cancer and are performing unnecessary hysterectomies as a result, say scientists.

They said a common indicator of tumours could actually show false-positive results leading to surgery that women do not really need.

Detection of human chorionic gonadotropin (hCG) in blood or serum is the standard test for diagnosing malignant trophoblastic disease, or choriocarcinoma - a rare cancer of the placenta.

Trophoblastic disease causes growth of a mole which can be harmless or can keep growing and, if left untreated, can become embedded in organs including the womb, and even spread via the blood to other parts of the body such as the lungs, liver or brain.

But research at the University of New Mexico on 12 women diagnosed as having the cancer suggests the procedures they underwent - hysterectomies or chemotherapy, and also in some cases removal of the ovaries - were unnecessary.

False-positive

Professors Laurence Cole and Sigi Rotmensch at Yale University School of Medicine said the false-positive results of the tests were due to the presence of human antibodies, called heterophil antibodies, which are present in a large number of people who do not have cancer.

They suggest that testing the serum of patients alone is not enough and the false positive results could be avoided by adding urine tests as well.

In a report published in The Lancet medical journal they said: "When medical diagnoses and therapies depend so heavily on one laboratory test, it is the clinician's responsibility to interpret results with caution."

They add: "Concurrent serum and urine testing for hCG should therefore be adopted as routine policy in the examination of persistent trophoblastic disease to avoid false-positive diagnosis of post-gestational choriocarcinoma."

Commenting on the study, Kenneth Bagshawe, of Imperial College School of Medicine, London, said: "In a litigation-conscious world, ensuring that clinicians making critical decisions based on immunometric assays for hCG are aware of the limitations could well merit more attention from laboratory managers."