Emerging as one of the key elements in the case for the defence of Michelle de Bruin yesterday was the difference in the specific gravity measurement of the swimmer's sample when measured in Kilkenny and measured some days later in Barcelona. It is the sort of scientific byway with which those following the case will become accustomed.

In her press conference on Tuesday De Bruin pointed out that the specific gravity reading on her urine sample when it was taken by tester Al Guy in her house on January 10th was 1.015. When tested several days later in Barcelona, the specific gravity had a reading of .983 g/ml.

Speaking for the defence over the past couple of days, having materialised from his job in Sky News, where he is a sports presenter, was Mark Saggars, who shed much sound but little light with his claims for the significance of the specific gravity issue.

Specific gravity is a measure of the heaviness of a substance compared to that of water and it is expressed without units. If something is 5.5 times as heavy as an equal volume of water its specific gravity would be 5.5.

Dr Don Catlin, the director of the US Olympic Committee's testing laboratory in Los Angeles is one of the world's leading experts on urine testing and dope control.

"There are two usual methods of measuring the specific gravity. One is the dipstick which is generally used by most officers in the field. The other is the displacement method used in laboratories. Recently in Nagano for instance a new tool called the refractometer has been added to the procedures approved. In general, though, sampling officers use the dipstick method."

"There are no written guidelines regarding the occurrence of discrepancies. It would be rare for a discrepancy to be relevant. Given the conditions of a laboratory test it would usually be considered that the lab test was more accurate.

Dr Catlin declined to comment on the possible effect of alcohol on the specific gravity of urine over a period of time. "In the current environment that would not be appropriate to comment upon," he said.

"I don't know of cases where the differences have been particularly relevant. I know of a lot of different cases I have experience in my own country where the difference has been easily explained. If there are scientists on the panel they will usually understand the reason for any difference."

The first thing which will have to be determined by the FINA doping commission will be the method of measurement used by Al Guy when he visited the de Bruins' house in Kilkenny and the method of measurement used on the same sample in Barcelona.

Of the two techniques generally available it seems likely that Al Guy, who was unavailable for comment yesterday, used the dipstick technique, commonly practised by testers working `in the field'. Laboratories commonly use a technique known as displacement measurement. If two different methods were used the discrepancy in the data is likely to be of little significance.

A significant fall in specific gravity between first measurement and second could be would be an indication of tampering somewhere in the chain of custody (Al Guy having sealed the samples in the presence of Michelle de Bruin in Kilkenny), probably involving the addition of water or alcohol. However, the relatively low figure given in the Kilkenny sample for the specific gravity of de Bruin's sample is unlikely to be a factor in deciding the case.

Guidelines on the issue in the regulations used for out-of-competition drug testing suggest the following:

"The specific gravity and pH of the urine shall be measured using a residual volume of urine in the collecting vessel. A specific gravity of 1.01 or higher is desirable. The pH should not exceed 7.5. If the sample does not meet those specifications, further samples may be required."

"Samples which do not meet the required specific gravity or pH should be processed, sealed and documented in accordance with normal procedures. Any subsequent sample collected from the athlete on this occasion shall be processed in the normal manner and documented on the same form."

De Bruin has also raised the issue of the small amount of urine which was forwarded to the laboratory in Barcelona. In her statement on Wednesday she said: "It should be noted that FINA's own regulations require that approximately 75 ml of urine is passed in order for a proper sample quantity to be made available to the testing laboratory. Given the circumstances that I have already outlined, the quantity of urine I passed for the sample was 70 ml."

Once again, however, regulations appear to allow for discretion.

"It is recommended that the A sample be of at least 40 ml and the reserve B sample be of at least 30 ml. However any shortfall in the amount of urine provided shall not invalidate a test."

Dr Catlin feels that the quantity of urine collected is unlikely to be an issue in any case.

"There is no minimum amount of urine which must be collected. There is a line on the bottle which says `fill to here' and sampling officers are trained to fill to beyond there, but sometimes in the field it is difficult to gather enough urine, so whatever can be gathered is sufficient."

Alcohol consumption has been used as a defence in cases of questionable testosterone/ epitestosterone ratios and the addition of a small quantity of alcohol to a sample would buttress a claim of altered ratio due to drinking. De Bruin has not claimed that this is the case and reports emanating from the IOC yesterday suggested that the level of alcohol involved would have killed most humans.