AFTER TAKING a long hard look at the District's preventive detention statute, the D.C. Court of Appeals declared the measure constitutional last week by a comfortable 8-to-1 margin. The decision was a timely one, coming only weeks after U.S. Attorney Charles Ruff had announced he would be using the law more frequently. Already this year he has requested preventive detention for 13 suspects, compared with a dozen for all of 1980.

The sole dissent came from Judge Julia Cooper Mack, who denounced preventive detention as unconstitutional and comparable to Lewis Carroll-style justice: "No!"No! Sentence first -- verdict afterward."

The court's majority, however, led by Chief Judge Theodore R. Newman, upheld detention for a period up to 60 days prior to trial when the prosecutor shows "clear and convincing evidence" (the statute's language) that: the detainee has been charged with a dangerous crime; his past behavior makes pretrial release a threat to public safety; and there exists a "substantial probability" that he committed the crime.

Judge John M. Ferren, who agreed that the law is constitutional, complained that Judge Newman had abandoned that "clear and convincing" standard of proof in favor of a less stringent "probable cause" standard. What Judge Newman and his colleagues did, according to his decision, was to endorse the existing standards that the government has applied for the last decade. Judge Ferren argued that defendants should also be allowed to cross-examinine witnesses and should have other procedural rights that would make detention hearings comparable to parole or probation violation proceedings. The majority said such rights do not properly belong in these hearings.

Should those rights be there? The record of restraint in the use of preventive detention by the U.S. attorney's office suggests that even without them abuses of the law are likely to be rare. But we can't help remembering that judges in these hearings are being asked to jail people who have not yet been convicted. Before jailing the suspects, the judges should be pretty well convinced those suspects are guilty, and the government should not be stingy with those procedural rights that may help an innocent suspect should the police arrest the wrong person. That's why we think those hearings are a good idea.