C. McClain (Mac) Haddow, 34, a high-ranking aide to Health and Human Services Secretary Margaret M. Heckler, has been chosen to succeed George W. Siguler as Heckler's chief of staff. Siguler left Dec. 1 after two years to become executive vice president of Monarch Capital Corp. of Springfield, Mass. Haddow was a Republican member of the Utah House from 1980 to 1982, and once worked in Utah for Sen. Orrin G. Hatch (R), chairman of the Senate Labor and Human Resources Committee. Haddow is distantly related by marriage to Hatch and has often said that Hatch's father was like a second father when his own father died.

READMISSION BLUES . . . Health policy analysts at John Hopkins University are warning that hospitals may try to get around the Reagan administration's prospective payment system by readmitting Medicare patients soon after they are discharged. As a result, they say, efforts to rein in hospital costs could be undermined.

Drs. Gerard F. Anderson and Earl P. Steinberg based their warnings on a study of Medicare records for 1974-77, which showed that 22.5 percent of all Medicare hospital patients were readmitted within 60 days of discharge, and that these subsequent admissions cost $2.7 billion a year in 1977 dollars. Projected forward to 1984, the same rate of readmissions would cost $8 billion, about a fifth of the total cost of the hospital insurance program.

In an interview, the two said that the new prospective payment system, under which hospitals receive the same payment for patients with a given illness regardless of the severity of their cases or the length of their stay, contains incentives for hospitals to step up readmissions even more.

Since the hospital can earn no more by keeping a patient longer, the hospital will try to push him out as quickly as possible. In some cases, the patient will have to be readmitted -- giving the hospital a second payment -- because he had not recovered sufficiently when he left. In addition, they said, the new system could discourage hospitals from treating several illnesses during a single hospital stay. It would be more profitable for them to treat each condition during a separate stay and obtain a payment for each.

Anderson and Steinberg also found that half of all Medicare patients are readmitted within a year of discharge.

A PENNY SAVED . . . HHS expects to save at least $3.7 billion in the future because of investigations or changes based on recommendations by HHS Inspector General Richard P. Kusserow in fiscal 1984.

For instance, Congress has lowered the reimbursement rates for laboratory fees under Medicare and Medicaid, for a projected savings of $974 million over five years.

It also voted to prohibit companies that purchase hospitals from revising the value of the hospital upward in line with their acquistion costs, which would have resulted in automatic increases in Medicare and Medicaid payments. HHS expects to save $635 million over five years through this change.

In fiscal 1984 there were 900 successful prosecutions of people attempting to defraud HHS programs, and another 400 convictions by state Medicaid fraud control units. In addition, 327 providers and suppliers were removed from the Medicare and Medicaid programs.

DIVORCED SPOUSES . . . Women's groups are warning that divorced people 62 or over who want to take advantage of a new Social Security provision cannot just sit back and wait for their checks. Under the new law, divorced persons can qualify for spousal Social Security benefits even if their former mate has not yet retired. But potential recipients must apply for the benefit; they will not be notified automatically that they are eligible.

Under the old law, a person married for at least 10 years and then divorced was eligible for the spousal benefit, based on the previous mate's earnings record, but only after the former husband or wife began drawing benefits of their own. This will all change Jan. 1. A wife divorced at least two years can start drawing the spousal benefit at age 62, regardless of whether her former husband has retired, as long as he is also at least 62. Former and current spouses are eligible to receive the same amount from the government.