Methodological issues in linking study participants to Australian cancer registries using different methods: lessons from a cohort study

OBJECTIVE: To compare results of concurrent linkage of records from the same occupational cohort to cancer registries at both a State and national level. Methodological issues affecting the record registry linkage process will be explored in cases of discordant results between the State and national cancer linkages. METHODS: The number of incident invasive cancers in an occupational cohort of more than 11,000 workers was determined by linkage to the National Cancer Statistics Clearing House (NCSCH) in 2003. The results were then compared with linkages by the cancer registries of Victoria and Western Australia in 2004, and also with a previous NCSCH linkage in 1998. RESULTS: Our analyses show an under-ascertainment of confirmed cancer cases by the NCSCH of about 13% (26/205) in Victoria and 11% (32/297) in Western Australia. In addition, 14 cases (5%) identified in a previous NCSCH record linkage in 1998 were not matched in 2003. CONCLUSIONS: These findings strongly indicate that record linkage to the State cancer registries was essential to maximise the ascertainment of cancer cases in our cohort. We attribute the discordant linkage results to differences in the quality of the record linkage process by the cancer registries. IMPLICATIONS: The record linkage methods of the national and State cancer registries need further standardisation. At present it is advisable to perform cancer record linkages through the State registries in addition to or instead of the NCSCH