Tainted China milk "may pose further kidney danger"

HONG KONG (Reuters) - Melamine, a plastic-making chemical that has been found in a growing number of Chinese milk products, may cause far more serious complications than just kidney stones, medical experts warned on Monday.

While stones can be removed easily, far more worrying is when melamine starts crystallizing in small kidney tubes, or tubules -- long, winding structures that filter blood, reabsorbing what is needed and excreting the rest as urine -- which will block connecting ducts and result in kidney damage and even failure.

"Human urine seems to have the right acidity for melamine to crystallize," explained Daniel Chan, professor in nephrology at the University of Hong Kong.

"The damage may be occurring in renal tubules where the urine is being transported. When a lot of the tubules are blocked by crystals, the kidneys don't serve their function and the children go into kidney failure," Chan said in an interview.

Melamine turned up last year in Chinese pet food exported to the United States, where many animals developed kidney failure and died. That scandal led to the theory melamine can crystallize in kidney tubules.

Wong Kar-yin, a consultant pediatrician in Hong Kong, added: "Large stones can be removed. But if renal tubules are stuck with crystals, there may not be effective treatment. Hopefully, other undamaged tubules can compensate for the function."

He said affected children would need long-term monitoring.

"If there is severe tubular damage, there could be kidney impairment ... the child may have to undergo dialysis or even need a kidney transplant. That's the worst case scenario," Wong said.

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Nearly 13,000 Chinese infants have so far landed in hospital after being fed milk formula that had been tainted with melamine. Four have died and more than 80 percent of the sick are under two.

Melamine is rich in nitrogen and was found to have been added to watered-down milk to fool quality checks, which often use nitrogen levels to measure protein.

Chan said the youngest children were most vulnerable.

"Damage seems highest in very small children who depend solely on milk for food. They consume large amounts and their blood level of melamine would be high," he said.

"When the kidney fails, it fails to clear the body of excessive electrolytes, like potassium. If blood potassium rises to a very high level, the heart will stop."

China needs to tell the world how the four children died -- whether from stones or tubular obstruction, the experts stressed.

While normal stone obstruction would take weeks, if not months, to lead to kidney failure, deterioration from intratubular obstruction would be far more rapid.

"For intratubular obstruction, kidney failure can occur overnight because it would be all over, not one or two tubules," Chan said. "The kidney is suddenly switched off ... toxins build up. That is more dangerous because it is more rapid."

(Reporting by Tan Ee Lyn; Editing by Nick Macfie and Alex Richardson)