Unless Congress acts to prevent it, elderly and disabled people without private insurance will have to pay at least $492 every time they enter a hospital, an increase of 23 percent over current out-of-pocket costs for Medicare beneficiaries. Curiously, the increase is a side effect of reforms that are producing hefty savings for the government -- a result that Congress probably didn't fully anticipate when it put the reforms into law.

The Medicare law has always required beneficiaries admitted to a hospital to pay an amount based on the average cost of a day of care for Medicare patients. As hospital costs have soared so has the so-called Medicare deductible -- this year it is $400, more than four times its level 10 years ago. But the scheduled jup in the deductible to $492 next January is not related to past or expected increases in costs. Quite to the contrary, it results from the fact that the length of hospital stays for the elderly -- and hence costs to Medicare -- declined dramatically between 1983 and 1984, the years upon which the coming adjustment is based.

During this period hospitals were adjusting to the new Medicare system, which pays hospitals a fixed amount for each type of ailment treated. To keep costs down, doctors were encouraged to discharge patients as quickly as possible. That reduced the total bill for a stay, but it also tended to inflate the cost for an average day of care, since hospitals had to recover certain fixed costs over fewer days of care. Since the Medicare deductible is based on average daily charges rather than total costs of a hospital stay, it has risen sharply.

The fact that the new Medicare payment system would tend to inflate the patient deductible was recognized by health planners in the administration the time the new system was adopted. But it is probably safe to say that they did not expect the rise to be so dramatic -- certainly Congress didn't. Sen. Edward Kennedy and others are urging the Finance Committee to consider moderating the increase before it is too late to stop it.

It is not unreasonable to ask elderly people to share more of the costs of a medical system that has provided enormous advances in diagnosis and treatment, especially at a time when the federal budget is under unparalleled strain. As Congress moves to narrow future deficits, additional contributions will almost surely be required. Still, there is a case for asking Congress to make sure that increases are gradual and predictable rather than abrupt and unexpected.