ATHLETICS: While much confusion and controversy continues to surround the use of nandrolone as an illegal anabolic steroid in world sport, there is little debate on the accuracy of the testing method.

When athletes are tested for the steroid, the scientists in fact search for nandrolone's waste product - 19-norandrosterone. If this is detected in urine it is regarded as proof that nandrolone has been present in the athlete's body. But because the substance also occurs naturally in the human body it has been necessary to establish acceptable levels.

For men, the limit is two nanograms of the waste product per millimetre of urine. For women the limit rises to five nanograms per millimetre. Those levels were also agreed upon as the acceptable standards by the International Olympic Committee.

On that basis the testing procedure holds no particular mystery. The problem is that nandrolone can appear in those test results for several different reasons - some legal, and some not so legal.

Most of the controversy in recent years surrounded sprinters, most notably Linford Christie and Merlene Ottey. Christie was said to have had more than 100 times the acceptable level of nandrolone in his system, but he later claimed his samples hadn't been properly refrigerated, and was cleared by UK Athletics of any wrongdoing - as was Ottey.

German distance runner Dieter Baumann was also cleared by his federation after claiming his toothpaste had been injected with the substance. Several other high-profile sports were effected by the controversy, with Czech tennis player Petr Korda and French footballer Christophe Dugarry also getting caught in the so-called nandrolone net.

Among the most popular explanations cited by athletes that have revealed positive tests is that the nandrolone in their systems arose from the use of protein supplements or the amino acid creatine, and other legal food supplements.

A recent investigation at Aberdeen University found that dietary supplements in combination with increased levels of exercise can produce a positive nandrolone test.