A contaminated urine sample is where the urine sample cannot be analysed due to it being mixed or contaminated with bacteria other than from the bladder (e.g. from the urethra, unsterile sample containers etc). Contamination is not the same but can lead to what is called a false positive or a false negative sample. The reason for this is that if the sample is contaminated the doctor cannot tell what is wrong and will make a judgement call believing that there is an infection (i.e. a positive) and prescribe medication or treatment when in fact there is no infection hence a false positive diagnosis. A false negative is when the doctor decides not to treat when in fact there was an infection and should have been treated. If it’s a false negative it could have very serious if not fatal consequences without treatment for example amongst ante-natal patients.