Objectives The Patient Survey provides basic information on disease and injury statistics of patients in Japan, and an estimation of the number of patients by disease and injury can be made using this survey. In this survey, the number of outpatients with repeat visits affects the survey results. The average interval since last visit (AILV) and a correction factor are used to estimate the number of repeat outpatients. Patients with AILV > 30 days are not included in the survey. However, in the last years, AILV exceeded 30 days in many cases, suggesting that the current 30-day threshold is no longer suitable. Thus, this study investigated the AILV in the current patient population and the effect of the increase in AILV on the number of repeat outpatients.Methods Patients Survey data of 1996-2011 were used to estimate the effect of changing the AILV threshold on the number of repeat outpatients.Results AILV increased for patients with most diseases and injuries. Using the current 30-day threshold, the overall outpatient coverage rate decreased from 91% in 1996 to 78% in 2011. A higher AILV threshold was necessary to maintain the overall outpatient coverage rate. For example, a threshold of 90 days increased the coverage rate in 2011 to 96%. However, raising the threshold markedly increased the number of repeat outpatients. For example, the overall number of repeat outpatients in 2011 increased from 43.01 million with the current 30-day threshold to 71.03 million using the 90-day threshold. The peak of the AILV of outpatients was observed on the next day after the first visit and the peak of the AILV of outpatients was observed every other week.Conclusion AILV increased over time and changing the AILV threshold markedly increased the number of repeat outpatients and total patients, indicating that there is a need to raise the AILV threshold.
Keywords: Patient Survey; average interval since last visit; estimated number of follow-up visits of outpatients; estimated number of patients; estimated number of patients receiving medical treatment.