Understanding changes in health status. Is the floor phenomenon merely the last step of the staircase?

Previous studies have found that health-status measures may be unable to detect clinically important changes for patients whose baseline health is poor (the "floor phenomenon"). It is not known whether this inability to detect change is confined to patients in very poor baseline health or whether the sensitivity of health-status instruments varies across the entire range of health states. The goals of this study were to see how changes in physical health, mental health, and overall health (1) depend on baseline (usual) health and (2) compare with patients' global assessment of changes in their health.

Stable, ambulatory patients presenting to the emergency department of a public hospital retrospectively rated their usual physical health (eight items), mental health (three items), and overall health (one item); their health on the day of study entry using these same items; and their global assessment of the change in their health compared with baseline. Complete information on these items was available for 1,005 patients. Baseline scores on the physical and mental health subscales and the overall health item were divided into five categories: 81 to 100 (best), 61 to 80, 41 to 60, 21 to 40, and 0 to 20 (worst).

The mean difference in health from baseline to emergency department presentation decreased as the baseline health category worsened, as follows: physical health, -26.0, -35.9, -15.1, -9.5, +1.0; mental health, -23.0, -16.1, -9.6, 0.0, 6.6; overall health -64.0, -45.3, -28.4, -8.4, 10.4, respectively. However, patients' global assessment of health change showed the opposite trend; the proportion of patients rating their health as "much worse" than baseline increased as baseline health worsened. When only patients whose physical health score declined less than 10 points were analyzed, 14% of those in the best baseline health said their health was "much worse," whereas 74% of those with the worst baseline physical health said their health was "much worse" than baseline.

These findings suggest that the sensitivity of health-status measures to change and the meaning of an incremental change in physical health or mental health vary depending on baseline health. This may result from noninterval properties of response options or from patients being at the lowest health state (the "floor") of individual questions. If health-status measures similar to this are to be used to compare the outcomes of treatment across diseases and for patients in a wide variety of baseline health states, weighting schemes may be needed to account for these effects.