Service planning implications of estimating Primary Health Organisation enrolment rate based on a Health Service Utilisation population rather than a Census-derived population.

Estimating Primary Health Organisation (PHO) enrolment rates with a census-derived estimated resident population denominator may provide misleading results because of numerator and denominator mismatch. This study uses the Health Service Utilisation (HSU) population denominator as an alternative.A HSU population was generated by record linkage of routinely collected datasets from the Ministry of Health via encrypted National Health Index (NHI). We compare PHO enrolment rates by age and ethnicity in Counties Manukau District Health Board (CMDHB) in 2013.In CMDHB, 98% of people who had utilised publicly-funded health services in 2013 were enrolled in a PHO in 2013. Using the HSU population as a denominator, PHO enrolment rates for Maaori, Pacific, Asian, New Zealand European/Other population groups were 98.3%, 97.7%, 97.6%, and 98.3% respectively. Just under 4% of people discharged from CMDHB inpatient facilities were not enrolled in a PHO within a month from the day of discharge in 2013.Using the HSU population as a proxy of health services need, PHO enrolment rates were similar across ethnicities in the CMDHB population. Support to improve PHO enrolment coverage would be more efficient if the HSU population were used to target people who are not yet enrolled in a PHO.