Indirect Estimation of the Number of Prevalent Cases – Relaxing the Steady State Assumption

Background: Prevalence is an important measure of disease frequency, helping characterize disease burden and health care needs. If direct estimates are unavailable (e.g. from surveys), the number of prevalent cases can be estimated indirectly as the number of incident cases per unit time times the average duration of disease. This formula, however, is based on a steady-state assumption which is often unrealistic, potentially precluding use of this indirect approach. Methods: We derive and illustrate use of an alternative formula relating numbers of prevalent cases, numbers of incident cases over time, the average disease duration and the mean square duration. Results: Examples illustrate use of our formula and that results are somewhat robust to the number of time periods used to estimate the linear trend in case incidence. Conclusion: Our formula does not require a steady-state assumption and applies provided temporal changes in incident cases are approximately linear.