PMC patient severity scale: derivation and validation - Patient Management Category

Health Services Research, August, 1994 by Wanda W. Young, Susan Kohler, Jeanne Kowalski

ISSUES IN THE DERIVATION OF RELATIVE SCORING SYSTEMS

The categories in a diagnosis-based classification represent a nominal form of measurement. To use such a classification effectively in comparative analyses across different populations and to adjust these populations for expected differences among patients with respect to their illnesses, it is often necessary to aggregate these patient categories into some form of composite measure. Such a measure can be based on one or more of a number of dimensions: severity, intensity of resource use or need, duration of days required for treatment, and actual or expected costs are a few of them. Depending on the basis of the index, resources (e.g., days, services, or costs) required to manage diverse patient populations can be projected and outcomes (e.g., morbidity or mortality) can be estimated.

Given the variety of measures possible, the decision to use a particular relative value scale should be guided by its intended use (The Hospital Research and Educational Trust 1989). Common practice among health care researchers, however, has been to use severity measures to adjust hospital expenses, assuming that "severity of illness," a concept that is generally not very well defined, is directly related to what it "should cost" for effective care. That is, researchers and practitioners have assumed that the more severely ill a patient is, the higher the costs should be.

Although severity and intensity of resource use and costs are directly related in many instances, in some disease groups this is clearly not the case. For example, an AMI patient with cardiogenic shock is one of the most severe PMCs (with an expected in-hospital death rate greater than 80 percent), but not one of the most costly cardiac patient types. Not only do patients with this condition frequently die early in their hospitalization, but those who are discharged alive have typically been managed medically (with or without angioplasty as opposed to open heart surgery). Other AMI patients with a lower in-hospital death rate (e.g., AMI with congestive heart failure) are more costly because they are more likely to receive major operative procedures. Thus, it should not be assumed for a particular disease that severity per se will necessarily be directly related to costs, charges, length of stay, or any other measure of resources used in patient management.

Despite the development of numerous indexes that purport to measure severity of illness, much ambiguity remains about the assumptions underlying the development of these measures (Stein, Gortmaker, Perrin, et al. 1987), as well as their construct and predictive validity. Not even the definition of severity is as straightforward as one would think given the commonly held belief that severity of illness is an important factor in determining patient morbidity and mortality (Thomas and Longo 1990).

Physicians seem to agree intuitively that the more severe clinical conditions are those associated with a greater probability of immediate death or disability and/or whose management (diagnosis and treatment) is more complicated than that for other conditions.(4) As part of the development of PMCs, physicians recognized severity distinctions among clinically distinct patient types within a disease and were able to rank these patient types by degree of severity. For example, a diverticular disease inpatient with non-massive gastrointestinal bleeding was considered less severe than a diverticular disease patient hospitalized because of an obstruction or fistula. These types of rank-order judgments within a disease have been incorporated into the modular hierarchy of the computerized PMC classification (Young 1984) and in other disease classification systems as well (Gonnella, Hornbrook, and Louis 1984).