Urinary dipsticks are not a good method of detecting an active urinary tract infection (UTI), despite widely being used by practitioners. Dipsticks can miss up to 70 per cent of infections, while cultures miss 50-80 per cent of UTIs.
If you feel symptoms of a UTI, but your tests are coming back negative, you need to sit your doctor down and give them this research: Reassessment of routine midstream culture in diagnosis of urinary tract infection.
Use of dipstick urinalysis for diagnosing an acute UTI
Researchers concluded that despite official guidelines and widespread use, these tests cannot be considered appropriate for diagnosing UTI in patients with lower urinary tract symptoms, and should be abandoned.
Use of standard culture for diagnosing an acute UTI
Midstream urine culture into a sterile jar is the main way that a sample for culture is taken away for testing. Researchers have found that people with lower urinary tract symptoms that are below the threshold of diagnostic cutoff for these cultures may have lingering bacterial colonies. Once the antibiotics are finished, the symptoms come back.
An evaluated was performed regarding the effectiveness of this test for detecting urinary pathogens according to the United Kingdom’s standard criteria for diagnosing an acute UTI. The results were disappointing. The culture failed to distinguish between asymptomatic controls and the symptomatic patients, and did not detect a variety of microbial species, including recognised uropathogens.
When the samples were run through DNA testing, the two groups were identifiable, and generated a more complete picture of what microbes were in the urinary tract.
Researchers concluded that the regular UK midstream urine culture misses a significant portion of bacteria, including common urinary tract pathogens, and may be unsuitable for excluding UTI in patients with lower urinary tract symptoms.
How standard midstream cultures work
Midstream urine culture is the gold standard for diagnosing acute UTI, with a specific recommendation on what colony forming units (cfus) are to reach threshold for a diagnosis. This threshold, of ≥105 CFU/ml was based on one study of 74 pregnant women with kidney inflammation and 337 asymptomatic women in the late 1950s. First, it’s old news, and second, it was a study on pregnant women only, who are known to have altered microbiomes to a certain degree during pregnancy.
This cutoff has been questioned, and a new recommendation of 102 CFU/ml was put forward as more appropriate. But, culture as a single measure, may not be appropriate when looking at the different ways that urinary tract infection presents, for example inflammation and symptoms appearing in the urethra, bladder, and/or kidneys. Many factors make it likely that no single threshold is going to suit every presentation of UTI, and that this strategy needs an urgent rethink.
Bacterial strains vary in how virulent they are, with some strains causing symptoms at lower concentrations, while other strains of the same bacteria need a significant amount more to cause symptoms. This can mean that some bacteria misses the threshold, and is therefore not diagnosed.
Urine concentrations vary wildly depending on how much a person has had to drink before the midstream sample is taken. This may also contribute to an insufficient number of bacteria present to meet threshold.
Another important factor is that certain bacteria, like Escherichia coli and Enterococcus faecalis can adhere to and invade cells, forming intracellular colonies that are undetectable in the urine.
Our innate immune response causes accelerated shedding of the urinary epithelial cells into the urine.
Mixed growth cultures are often dismissed as contamination by normal microbes that colonise the vagina, periurethral and perianal areas. This leads to cultures being tossed out as useless. The issue here is that polymicrobial infections are common in people with lower urinary tract symptoms.
E. coli has very crafty invasive techniques when isolated from a culture with many other bacteria in it (polymicrobial), than when it was found on its own. This is important, because it seems to indicate that E. coli gets more aggressive when amongst others than when it plays by itself. E. faecalis and E. coli support each other with nutrients (I-ornithine).
- Khasriya R, Khan S, Lunawat R, Bishara S, Bignal J, Malone-Lee M, et al. The Inadequacy of Urinary Dipstick and Microscopy as Surrogate Markers of Urinary Tract Infection in Urological Outpatients With Lower Urinary Tract Symptoms Without Acute Frequency and Dysuria. JUrol. 2010;183(5):1843-7.
- Kupelian AS, Horsley H, Khasriya R, Amussah RT, Badiani R, Courtney AM, et al. Discrediting microscopic pyuria and leucocyte esterase as diagnostic surrogates for infection in patients with lower urinary tract symptoms: results from a clinical and laboratory evaluation. BJU Int. 2013;112(2):231-8. doi: 10.1111/j.1464-410X.2012.11694.x. PubMed PMID: 23305196.
- Gill K, Kang R, Sathiananthamoorthy S, Khasriya R, Malone-Lee, J. A blinded observational cohort study of the microbiological ecology associated with pyuria and overactive bladder symptoms. Int Urogynecol J. 2018. Epub 2018/02/20. doi: 1007/s00192-018-3558-x. PubMed PMID: 29455238.