Calculating Follow-up Rates

Follow-up rates are presented as a percentage but the method of calculation is seldom discussed. Determining a follow-up rate begins with identifying the numerator and denominator used in the calculation. Four methods of calculating follow-up rate after surgical treatment for weight reduction were studied. Method 1 follow-up rate, 91.2%, was calculated using patients with at least one follow-up contact as the numerator (6169) and the number of primary operations performed as the denominator (6764). Method 2 calculated patient contact on or beyond a specific time point: 65.9% for ≥ 6 months (4232/6424), ≥ 1 year (y) = 52.1% (3111/5977), ≥2 y = 29.8% (1455/4890), ≥3 y = 18.8% (710/3784), ≥4 y = 12.5% (331/2643). Follow-up calculated using method 3 determined patient contact within specific time points: 0-1 y = 89.7% (6064/6764), 1-2 y = 45.8% (2739/5977), 2-3 y = 23.0% (1124/4890), 3-4 y = 13.9% (526/3784), 4-5 y = 11.3% (298/2643). Method 4 studied patient contact within time points according to ASBS guidelines. Rates for method 4 were: 0-6 months = 58.8% (11,938/20,292), 6-12 months = 36.7% (4717/12,848), 1-2 y = 45.8% (2739/5977), 2-3 y = 23.0% (1124/4890), 3-4 y = 13.9% (526/3784). Results using these four methods of calculation emphasize the need to standardize follow-up rate before operative comparisons can be made.