One of the most common specimens processed in a microbiology laboratory is the urine sample, with hundreds of thousands being processed in each lab every year. You would expect that all doctors would know how to interpret the result of such a common sample type, however in reality this is often not the case.
Medical students often get taught methods for interpreting investigations such as chest x-rays but rarely do they get taught how to interpret laboratory tests. As a consequence significant results are easily missed and sometimes patients are treated for infections they don’t have because too much emphasis has been placed on a single part of the result. I have seen patients treated for recurrent urinary tract infections (UTIs) which are in fact multiple contaminated badly taken urine specimens (for more details on how to correctly take a urine sample see Blog Piddle in a bottle!). Not only did these patients receive multiple courses of unnecessary antibiotics but the doctors stopped looking for another cause of their symptoms which may have serious adverse outcomes if time to diagnosis is critical e.g. where cancer is concerned.
One of the biggest and most obvious mistakes is that doctors don’t look at the whole result, but jump to the culture section that states the organisms. Ask yourself, why would a lab tell you the other stuff (microscopy, culture and sensitivities) if they weren’t important?
For urine results the first part of the result is the microscopy. An understanding of this is required in order to interpret the culture result that follows. The microscopy result tells two main things:
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.
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.
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