President Carter's disavowal last week of his three-month-old confession that the invasion of Afghanistan changed his view of "ultimate" Soviet goals followed a familiar pattern of reshaping positions to his political needs, a syndrome newly apparent in his treatment of germ warfare charges against Moscow.

In an interview with The Washington Post's Meg Greenfield, Carter denied he had ever admitted changing his own mind about the Soviets' "ultimate goals" as a result of Afghanistan. In fact, on Dec. 31, following the Soviet invasion, Carter declared over ABC television that Afghanistan "made a more dramatic change in my opinion of what the Soviets' ultimate goals are than anything they have done" during his administration.

Thus, Carter's syndrome: what he says for immediate political impact one day, he denies the next under the stress of changed political need. The result is that U.S. policy becomes hostage to instant political requirements. That has infuriated U.S. allies, confoundedU.S. enemies and consigned Carter's real policy to the murky world of make-believe.

The Carter syndrome has been tripping him and his country in foreign policy for three years, but never so consistently as in these days of the Soviet global offensive and the Carter reelection drive.

On Jan. 23, one month after Afghanistan, the president sounded a call to arms in his State of the Union message. It is "imperative," he said, that Congress approve a 1981 defense bill equal to "a 5 percent real growth in authorizations, without any reduction."

When Carter's budget revisions reached the House Armed Services Committee March 31, he called for a 2 percent real growth. In just three months, the president's perception of political reality had caused him to downgrade drastically what he had told Congress was an "imperative" of military preparedness.

The case of alleged Soviet violations of the 1972 treaty banning biological (germ) warfare also has triggered the syndrome. When the allegations about violations were first heard by Carter last spring, he turned a deaf ear. He did so not because the suspicions had been disproved, but because he did not want to harm his cherished strategic arms limitation agreement (SALT II).

Any proof that Moscow has violated the insistent and specific terms of the germ warfare treaty would have made SALT II its first casualty. So, the wraps were on; the explosion in a suspected biological warfare plant in the closed city of Sverdlovsk, nearly 1,000 miles east of Moscow, was given the quick brush.

Afghanistan changed everything. Suddenly, the same circumstances deemed inadmissible for U.S. scrutiny only months earlier were perceived, said a State Deparment spokesman, as having caused "substantial numbers of fatalities perhaps running into the hundreds."

But will Carter insist tomorrow, as he does today, on uncovering the truth about the alleged Sverdlovsk treaty violation? The State Department says he will. The president's record of making policy according to political needs of the moment strongly argues that he will not.

That is particularly true in this case because a U.S. finding that the Russians blatantly cheated on the unverifiable germ warfare treaty would revolutionize the political climate here against the strategic arms agreement. The top-secret guideline drafted by the National Security Council's Special Coordination Committee last month, informing Carter the administration was "raising this matter" with Moscow, specifically noted "potentially serious implications for the future of arms control."

Those implications center not on germ warfare but on the broader question of nuclear arms control. In the words of one top arms-control expert, a finding of Soviet guilt at Sverdlovsk "would terrify the arms controllers, not just because of the verification problem but because it would prove Soviet intent to cheat at a time many arms controllers claim that there is no such intent."

Some day, perhaps soon, Jimmy Carter may decide to resum all-out pursuit of detente, despite Afghanistan. When that day comes, the Carter syndrome makes change easier: the Sverdlovsk incident, along with the 5 percent increase in the defense budget and many other now-forgotten Carter policies pegged to the passing fancy, will simply be denied and hidden in some musty file.