Tumour Markers: Same Marker, Different Assay, Different Result

Tumour markers are deceptively easy to use and are widely employed in medicine. They comprise a group of substances, elaborated from tumours, which may be used to diagnose, stage and monitor neoplastic disease. Laboratory specialists are generally of the opinion that clinical colleagues, through no fault of their own, are insufficiently aware of the pitfalls of tumour marker use and of the methods of best practice. In part this has arisen because of the success of laboratories in providing results in a way that masks the clinician from the complex technologies employed in routine analysis. The article by Hjiyiannakis et al. in this issue of Clinical Oncology once again presents data on a classic problem that is typical of those faced every day by laboratories that provide tumour marker services [1]. Thyroglobulin measurement has for nearly two decades been a mainstay in the follow-up of patients with differentiated thyroid carcinoma. Throughout that period, a debate has continued regarding the significance of the presence of circulating antithyroglobulin antibodies (TbAb) [2]. These occur with a prevalence of 10% in the normal population and of up to 25% [3] in patients with differentiated thyroid carcinoma. Because current analytical methods are based exclusively on immunological techniques, the possibility of assay interference by these antibodies is present whatever assay format is used [4]. The exact mechanism of interference varies between different assay formats, whether they be single or double antibody, whether the assay antibodies are monoclonal or polyclonal, and whether competitive or immunometric detection systems are employed. Essentially, however, the cause of the interference is competition for antigenic epitopes between the patient’s native TbAb and the antibodies in the assay system; further variability of effect arises owing to differences in the specificities and affinities of the TbAb from individual patients. Numerous