How many surgery appointments should be offered to avoid undesirable numbers of 'extras'?

Patients seen as 'extras' (or 'fit-ins') are usually given less time for their problems than those in pre-booked appointments. Consequently, long queues of 'extras' should be avoided.

To determine whether a predictable relationship exists between the number of available appointments at the start of the day and the number of extra patients who must be fitted in. This might be used to help plan a practice appointment system.

Numbers of available appointments at the start of the day and numbers of 'extras' seen were recorded prospectively in 1995 and 1997 in one group general practice. Minimum numbers of available appointments at the start of the day, below which undesirably large numbers of extra patients could be predicted, were determined using logistic regression applied to the 1995 data. Predictive values of the minimum numbers calculated for 1995, in terms of predicting undesirable numbers of 'extras', were then determined when applied to the 1997 data.

Numbers of extra patients seen correlated negatively with available appointments at the start of the day for all days of the week, with coefficients ranging from -0.66 to -0.80. Minimum numbers of available appointments below which undesirably large numbers of extras could be predicted were 26 for Mondays and four for the other week-days. When applied to 1997 data, these minimum numbers gave positive and negative predictive values of 76% and 82% respectively, similar to their values for 1995, despite increases in patient attendance and changes in the day-to-day pattern of surgery provision between the two years.

A predictable relationship exists between the number of available appointments at the start of the day and the number of extras who must be fitted in, which may be used to help plan the appointment system for some years ahead, at least in this relatively stable suburban practice.