The effect of lead time bias on severity of illness scoring, mortality prediction and standardised mortality ratio in intensive care - a pilot study

The effect of lead time bias on severity of illness scoring, mortality pred iction and standardised mortality ratios was examined in a pilot study of 7 6 intensive care (ICU) patients using APACHE II, APACHE III and SAPS II sco ring systems. The inclusion of data collected in the period prior to ICU ad mission increased severity oi illness scores and estimated risk of hospital mortality significantly for all three scoring systems (p < 0.01) by up to 14 points and 42.7% (APACHE II), 50 points and 26.3% (APACHE III) and 23 po ints and 33.4% (SAPS II), respectively Standardised mortality ratios fell f rom 0.99 to 0.79 (APACHE II), 0.96 to 0.84 (APACHE III) and 0.75 to 0.64 (S APS II), but these changes failed to reach statistical significance. Lead t ime bias had most effect in medical patients and on emergency admissions, a nd least effect in patients admitted from the operating theatre. These tren ds suggest that mortality ratios may not necessarily reflect intensive care unit performance and indicate that a larger study of the effect of lead ti me bias, case mix, pre-ICU care or post-ICU management on standardised mort ality ratios is indicated.