BEIJING (AP) — Quality inspectors have found no melamine in infant formula produced by U.S. drug maker Wyeth, despite claims from about 20 families that said the milk caused kidney stones in their children, a state news agency reported Thursday. Wyeth was the latest foreign maker of infant formula to be accused in China's tainted baby milk scandal, in which at least six infants were killed and almost 300,000 sickened. Numerous Chinese brands were found to be contaminated with melamine, an industrial chemical that can cause kidney stones and kidney failure when ingested. China's quality watchdog tested every batch of milk powder produced and imported by Wyeth since Sept. 14 and found no problems, the official Xinhua News Agency reported. The tests came after a group of about 20 families said their children developed kidney stones after drinking Wyeth milk, which had been deemed safe in the height of the crisis late last year. Madison, N.J.-based Wyeth has dismissed claims that its products are unsafe. It said its formula has been repeatedly tested and found safe by an independent lab as well as by the governments of China and five other countries in the region. A few dozen Chinese parents have also blamed baby formula produced by Dumex Baby Food Co. Ltd., a subsidiary of France's Groupe Danone SA, for sickening their children. Dumex has insisted that all its products are safe and tests by the Shanghai government have found no melamine contamination. China's Ministry of Health has asked local health bureaus to begin epidemiological research on kidney problems in children, including checking their eating habits and living environment. The tainted milk scandal, which unfolded in September, was one of the country's worst food contamination crises. It involved the products of China's biggest dairies and underscored the government's problems with policing product quality. Unscrupulous middlemen are accused of mixing melamine into watered-down milk to fool quality tests for protein. Two people have been sentenced to death for their roles in the contamination, and the former chairwoman of the dairy at the heart of the scandal was sentenced to life in prison.