January 8 (HealthDay News) -- Quest Diagnostics, the largest provider of medical laboratory tests in the United States, says it has fixed a problem that led to higher vitamin D readings for about 7 percent of patients from 2007 to 2008, the Associated Press reported Wednesday.

The Madison, N.J., lab said it noticed an "upward trend" in the vitamin D levels being registered on some of its tests during the summer, then offered free tests for patients whose results were called into question, according to Gary Samuels, the company's vice president for communications. Blood tests to check vitamin D levels are on the rise, because research has shown a possible link between too little "sunshine vitamin" and a higher risk for cancer and heart disease.

Quest's chief medical official, Dr. Wael Salameh, told AP that he doubted patients would have suffered any harm. People with serious vitamin D deficiency, he said, often exhibit physical symptoms such as fractures that doctors would have noticed. "A good doctor would question the test," Salameh added. "For the few vulnerable patients, other indicators would have flagged the situation to their physician."

Quest said the cause of the problem proved to be how some of the company's testing chemicals were mixed. But the company is using a new testing technology, AP reported, which critics say tends to produce higher vitamin D readings.