Objective: For registries that routinely delay case registration, the flow method does not accurately describe their completeness over time. A modification to allow for such a delay is proposed and tested.
Methods: Using original data set from Ticino Cancer Registry, Switzerland, a new data set (the delayed data set) was created by adding two years to the date of registration for each case in the original data set, to emulate the situation in a registry where registration is delayed by two years. Both the original and modified methods were then applied to both the original and the delayed data set.
Results: When applied to the delayed data, the original method produced estimates of completeness of 32 and 43% at one and two years after diagnosis. When the modified method was applied to the delayed data, the completeness at one and two years was correctly estimated at 0%. After the initial two-year time lag, completeness was consistently estimated by both methods. When applied to the original data, the modified method produced the same results as the original method.
Conclusions: The proposed modification allows the method to be applied even when registration is delayed long after diagnosis-thus extending the range of registries for which the flow method can be used.