1. Is there an inflammatory process going on?
UTIs usually cause inflammation and this is reflected in the presence of white blood cells in the urine.
NO white blood cells means - NO inflammation and therefore UTI is VERY UNLIKELY.
Remember, certain patient groups e.g. pregnancy, children, immunodeficiency and neutropaenia, complex urological anatomy, can have UTIs without white blood
cells and therefore the clinical background of the patient is important. Any bacteria that grow in the absence of white blood cells are likely to represent contamination from an incorrectly taken sample.
2. Is there a risk of contamination from the skin of the perineum?
The presence of epithelial cells in a urine sample indicates that it has been in contact with perineal skin. Epithelial cells indicate contamination. If a UTI is suspected, these urine samples should be repeated, ensuring the patient knows and follows the correct method for taking a urine sample, if possible before antibiotics are prescribed.
Once you have decided about whether any urine result is likely to be due to a UTI or contamination (by looking at the
microscopy) it is safe to look at the culture part of the result.
3. If bacteria grow in the presence of white blood cells and the absence of epithelial cells then UTI is LIKELY to be the cause of the patient’s infection. If the microscopy and culture is not consistent with this, UTI is not the likely cause and another source for the patient’s illness should be sought.
Remember, the perineal skin is not sterile, in fact it’s normal flora includes all of the bacteria that commonly cause UTIs e.g. E. coli, Proteus sp, Klebsiella sp. etc. If these bacteria get into the urine sample via contamination (presence of epithelial cells indicates contamination) then they will grow and give a false impression that a patient has a UTI.
Note: If the microscopy result suggests a UTI (WBC but no epithelial cells) but nothing grows on culture you should then investigate if there might be a reason why a bacteria is being prevented from growing e.g. the patient is already on an antibiotic, or whether there is another reason for a sterile pyuria e.g. a systemic inflammatory response putting white blood cells into the urine e.g. appendicitis, pneumonia.
It is the combination of the whole report, microscopy and culture, which ensures the correct interpretation of the result.
Looking at only one aspect in isolation will lead to misdiagnosis and poor patient management.
See section - Clinical Scenarios, UTIs for test yourself examples on how to correctly interpret a urine result.