The composition of human urine is very complex and influenced by the food taken prior to sampling. In all cases, urine represents a rich nutrient source for the growth of microorganisms. At a temperature between 5 and 40 °C and a modest pH (4.5–8.5) microorganisms can multiply at very high rates with generation times as low as 30 min. [5–7] As a result, a single bacterium can increase to 109 cells in 15 h.
In principle, urine in the bladder is sterile. It gets, however, contaminated with bacteria when it leaves the urinal tract and is collected in bottles, which are not sterilized. Initial contaminations can be in the order of 104cells/ml and hence it would take less than 10 h to reach 109cells/ml and thus spoil the urine sample completely.
Laboratory experiments, using urine samples from a number of individuals incubated at 20–30 °C, have proven that the described storage circumstances lead to significant microbial contamination. In each of the samples, substantial contamination, i.e., spoiling of the sample, was demonstrated within 16–24 h, as shown by visual and microscopic inspection ([6, 7] and experiments performed by one of the authors). In these cases also the pH had risen.
It must be stressed that a large diversity of microorganisms, bacteria, yeast and fungi can grow in urine. They posses an even much larger array of biological systems, including enzymes that can change the compounds present in the urine [4, 8]. A large number of publications have shown that specific enzymes can transform a multitude of hormones, drugs, antibiotics and xenobiotics . In essence, it is close to impossible to predict exactly the outcome of microbial degradation, transformation and even new synthesis of complex compounds, such as steroids [8, 9]. This can lead to false-negative and even false-positive doping tests . Therefore, it is obvious that microbial contaminated samples no longer represent the originally sampled urine and hence had lost their integrity. They are therefore not suitable anymore for further analysis. Indeed, it is unthinkable that any medical laboratory would ever accept contaminated urine samples for analysis.
The inescapable conclusion is that to maintain the integrity of urine samples, microbial growth should be inhibited. A recent literature study, financed by WADA, by Tsivou et al.  gives an excellent overview of different options. The conclusion of that study is that no other method other than cooling and storing at ≤5 °C is suitable. However, to our great surprise, it is accepted as a matter of fact in the same report that cooling or freezing during transport is not always possible in the daily practice of doping control. We challenge this fact as totally out of date and therefore scientifically and morally unacceptable (Table 1).