The nation’s largest medical laboratory company provided possibly erroneous results to thousands of people who had their vitamin D levels tested in the last two years, the company has acknowledged.

The company,Quest Diagnostics  has already sent letters to thousands of doctors listing the patients who might have received “questionable” test results and is offering free retests. The company said it had fixed the problems.

An erroneously high result may mean patients will not take vitamin D supplements when perhaps they should, doctors said. And an erroneously low test result might lead in rare instances to a toxic overdose of vitamin D. When the Quest tests have been inaccurate, the reading has typically been too high, although not in all cases.

Quest’s action represents “the largest patient test recall I’m aware of in my 20 years in the business,” said Robert L. Michel, editor of the dark report, a newsletter for pathologists that first reported on Quest’s action.

The incident could raise calls for more regulation of diagnostic testing at a time when diagnostics are playing an increasingly crucial role in guiding medical treatment. Many laboratory tests, including Quest’s vitamin D test, do not require approval from the Food and Drug Administration.

But experts say that so far there is no standardization of the tests, meaning that results can differ considerably from one laboratory to another.

Dr. Binkley said that a few years ago he sent a sample of his blood to six laboratories and got results that ranged from 14 nanograms a milliliter, which would be a deficient level, to 41 nanograms — a level three times as high and considered adequate. While the tests’ consistency has improved since then, there can still be substantial variability, he said.

Yet many experts say that even if the tests were accurate, there would still be uncertainty in how to treat patients. There is considerable debate about how much vitamin D is needed and whether vitamin D supplements even prevent various diseases. Vitamin D is usually made by the body when the skin is exposed to sunlight and is also contained in oily fish and some fortified drinks like milk.

Some doctors said they had switched to other testing laboratories because of Quest’s problems. But Quest, based in Madison, N.J., offers numerous tests, from routine blood work to sophisticated genetic tests. No one type of test represents a large potion of its revenue, which was $5.45 billion for the first nine months of 2008.

The company said the retesting would not have a material effect on its earnings. The company’s stock price rose 80 cents Wednesday to $49.20, approximately in the middle of its 52-week range.

Quest and many other laboratories argue that lab quality is already regulated through Medicare and that having to win approval for each new test will slow innovation and raise costs.

Non-melanoma skin cancer, formally known as basal cell or squamous cell carcinoma, is the most commonly developed skin cancer affecting 1 million people in the United States each year. It is often easily detected and, when treated properly, has a cure rate of 95 percent, according to the American Academy of Dermatology. This type of cancer rarely spreads to other parts of the body, but it can extend below the skin to the bone and cause considerable local damage. Doctors already know that non-melanoma skin cancer places people at high risk for developing additional skin cancers, but new research indicates it may also increase the risk of developing other malignancies in the body.

A team of researchers led by Jiping Chen, M.D., Ph.D., of the National Cancer Institute and Anthony Alberg, Ph.D., MPH, associate director for cancer prevention and control at the Hollings Cancer Center of the Medical University of South Carolina in Charleston looked at the data from a 16-year study involving 19,174 residents in Washington County, Maryland; 769 with non-melanoma skin cancer and 18,405 who were cancer free. After measuring and comparing the cancer risk of the participants, they found that individuals aged 25 to 44 who suffered from non-melanoma skin cancer were 2.6 times more likely to develop other types of cancer later in life, compared to those who never experienced the disease. In addition, the younger a person developed non-melanoma skin cancer, the higher their risk of developing other cancers.

Among the cancers likely to be contracted, melanoma was the most common, with eight times the risk, but there was also an increased risk of developing lung, colorectal and breast cancers. Prostate cancer was also linked but the association was not statistically significant. “It seems like non-melanoma skin cancer, even though it is a non-fatal disease, may be a warning sign for increased risk of other, more serious cancers,” Dr. Alberg said

Researchers speculate that part of the reason for the increased risk is that some people have difficulty repairing DNA damage, which make them more susceptible to environmental irritants and more likely to develop cancer. Furthermore, patients who have survived non-melanoma skin cancer are visiting their doctors regularly for follow-ups and are therefore more likely to be diagnosed with other cancers. “A personal history of non-melanoma skin cancer may be more a part of personal health history than we thought before,” said Dr. Alberg.

Margaret Karagas, associate director of the Center for Environmental Health Sciences at Dartmouth Medical School, told ABC News that this study could provide a “useful model” for future cancer research, listing radiation and arsenic among the cancer-causing agents that were initially discovered because they caused skin cancers. “It is also important because in many parts of the world it appears to be one of the most rapidly increasing cancers,” she said.

Another study led by Jennifer Christian from the VA Medical Center and Brown University found that patients taking blood pressure-lowering drugs called angiotensin-converting enzyme (ACE) inhibitors or angiotensin receptor blockers (ARBs) had a 39 percent lower risk of developing basal cell skin cancer and a 33 percent lower risk of developing squamous cell skin cancer. The researchers say it is unclear why these drugs lower the risk of skin cancer.

More than one third of mothers who chose to undergo a repeat caesarean section had their babies earlier than medical guidelines recommend, and the earlier the babies were born, the more likely they were to experience a medical complication like respiratory distress, a large study has found.

Although babies born by Caesarean section after just 37 weeks of  pregnancy are considered full-term, the study found they were twice as likely to suffer complications as those born by Caesarean after 39 weeks. Infants born by Caesarean at 38 weeks were 50 percent more likely to have a medical problem than those born at 39 weeks. The complications included respiratory distress, infections,hypoglycemia, being admitted to the neonatal intensive care unit, or being hospitalized for five or more days.

The study, funded by the National Institute of Child Health and Human Development, was published on Thursday in The NEW ENGLAND JOURNAL OF MEDICINE.

It is being released amid concern over the rising numbers of preterm births in the United States and the trend of many women and their physicians to schedule Caesarean deliveries, or C-sections, well in advance of the expected due date. Some 30 percent of deliveries were done by Caesarean section in 2006, up from 20.7 percent in 1996. About 40 percent of the 1.3 million Caesareans performed in the United States each year are repeat procedures, though not all are elective.

“There is an understanding that for a baby born at  term, the risks are really very low, and many people may just assume that outcomes at 37 weeks may be the same as at 39 weeks,” said Dr. Alan T.N. Tita, the paper’s lead author and an assistant professor of obstetrics and gynecology at the university of alabana at birmingham. “We know that is not true.”

He noted that the American College of Obstetricians and Gynecologists recommends elective Caesareans not be performed before the 39th week of pregnancy.

Medical problems also occurred more frequently in the small group of babies delivered by  Cesarean at 41 weeks and 42 weeks, the study found.

Researchers reviewed a registry containing detailed information about repeat Caesarean births to mothers who had previously had the procedure at 19 medical centers, all part of the Eunice Kennedy Shriver NICHD Maternal-Fetal Medicine Units Network. Of 24,077  Caesarean deliveries performed at term, 13,258 were elective, meaning there was no medical indication for the surgery, the researchers found. Of those, 6.3 percent were performed at 37 weeks and 29.5 percent at 38 weeks.