In an effort to help consumers comparison-shop for medical care, a coalition of employers and health care providers has rated 21 California health maintenance organizations on the quality of their preventive health services.

While HMO marketing campaigns typically tout preventive health services, or "wellness," to distinguish themselves from traditional medical plans, the report suggests that HMOs can differ sharply in how well they take care of their customers.

However, some HMOs that received high marks in some measures of medical quality performed less well in other areas, making it difficult to assess overall performance.

Only one HMO--TakeCare Inc., which was acquired last year by Fountain Valley-based FHP International--stood out from the pack, registering above-average marks in four of six measures. No other HMO received more than two above-average scores.

"You can't just look at the plans and grade them as 'A,' 'B' or 'C,' " said Patricia Powers, executive director of the Pacific Business Group on Health, an influential health care purchasing cooperative formed by major California employers. Her group was one of two major employer groups--the other was the California Public Employees Retirement System--that participated in the study. Other coalition members included the 21 HMOs and affiliated medical groups.

"You have to look across the plans and ask which one is doing a better job for the things I care most about," Powers said. "If you are a woman, you might care more about prenatal care or childhood immunization."

The quality report reflects growing demands by the purchasers of health care--mainly employers--to hold HMOs accountable for their claims of providing quality health care. In response, many health plans have produced their own report cards.

But the new study took a different approach--one that experts say is more credible. The coalition, the California Cooperative HEDIS Reporting Initiative, hired an independent health care consultant, the Medstat Group, to analyze information culled from 25,000 medical charts at more than 5,000 doctors' offices statewide.

The Medstat Group looked at medical data for 1993 in six areas: childhood immunizations, breast cancer screenings, cervical cancer screenings, prenatal care, cholesterol screenings and diabetes retinal exams. The 21 HMOs were ranked above average, average or below average for each area.