The median net income of office-based physicians rose 12.8 percent last year to $132,500, and the credit may go to the doctors themselves. They are becoming better business managers.

A survey by the magazine Medical Economics found the 1989 median income rose $14,770, the biggest dollar increase in the 61 years the survey has been conducted by the financial publication for physicians.

The article describing the study suggested higher incomes resulted more from better business practices, especially more efficient bill collection, than from a larger number of patient visits or higher fees.

It points out, in fact, that while median gross receipts rose sharply, much of the revenue "was gobbled up by a 19.8 percent rise in professional expenses -- the steepest such increase of the decade."

Nevertheless, the higher incomes could provoke controversy because they come at a time when business, insurers, patients, federal officials and the medical profession itself are seeking to hold down steadily rising costs.

The article points out that while the typical doctor "had lost six of his nine most recent yearly skirmishes with inflation, 1989 put him comfortably ahead for the decade."

While the consumer price index rose 64.4 percent from December 1979 to December 1989, the article states that doctors' "cumulative annual earnings growth for the period topped that figure by 8.4 percentage points."

However, doctors in family or general practice earned considerably less than the median, with the former grossing $211,540 and netting $97,340, and the latter grossing $170,423 and netting $86,860.

The study was based on questionnaires mailed in January to 31,976 medical doctors in private, office-based practice. By the mid-May cutoff date, 9,762 responses had been trimmed to a working sample of 6,484 questionnaires.

The magazine stated that the survey was representative in terms of physicians' field of practice, region and age and therefore "likely to be representative in other ways as well."

The responses showed cardiovascular surgeons as the highest paid of all medical doctors, with a gross income of $493,750 and a net of $296,300, followed by neurosurgeons at $455,560 and $256,600, respectively.

Orthopedic surgeons, plastic surgeons and ophthalmologists followed, all with net incomes exceeding $200,000. Obstetric and gynecological specialists, thoracic surgeons, anesthesiologists and ear-nose-throat specialists followed at just under $200,000.

Among other specialties, general surgeons were said to have netted a median of $143,130, internists $110,500, psychiatrists $103,570 and pediatricians $101,800.

The highest median net for medical doctors in general was found in the South Atlantic states, at $143,500, in the Mid-South at $143,300 and in the Far West, including Alaska and Hawaii, at $135,420.

In order below them were the Southwest states at $132,500; the Mid-Atlantic states of New York, Pennsylvania and New Jersey at $131,790; the Great Lakes region at $125,830; New England at $125,000; and the Great Plains at $123,750.

The survey found the highest gross and net incomes in Florida, at $282,500 and $157,950 respectively, representing a one-year increase of $34,820, or 28.3 percent.

In his commentary, Medical Economics Editor Don Berg stated that "this year could well be the last one of really big increases" before Medicare caps and other restrictions begin.