Comparison of the diagnosis of leukaemia from death certificates, cancer registration and histological reports - implications for occupational case-control studies.

It is essential in occupational case-control studies of rare diseases for ascertainment to be as complete as possible, together with an accurately defined diagnosis. A nested case-control study from a large cohort of UK oil distribution workers followed up since 1950 was carried out to investigate the association between leukaemia, in particular acute myeloid leukaemia, and exposure to benzene. Ninety-one cases occurring before 1993 were identified from death certificates or cancer registrations (available from 1971). Histopathology departments were contacted to obtain material that might confirm the diagnosis of leukaemia and this was received for 39 (43%) cases. The majority of the cases (88) were identified primarily from death certificates, with a cancer registration also being received for 56 (90%) of the 62 deaths occurring after 1971. Discrepancies in the diagnoses from these two sources were found for 12 cases, five being acute myeloid leukaemia. For the majority, the diagnosis on the death certificate was more specific than that on the cancer registration. Histology reports were received for nine of the discrepancies, all confirming the death certificate diagnosis. Although leukaemia appears to be regularly registered as a cancer, records may not be routinely updated when new clinical information becomes available. It is recommended that death certificates, cancer registrations and histology reports are obtained routinely by cancer registries to maximize both numbers of cases and diagnostic accuracy for epidemiological studies.