Urinary Tract Infection Tests — what they are and what they mean (or don’t mean)

<- Back to Learning Center

<- Back to Learning Center

Spencer is a Co-Founder of Uqora. Trained in biochemistry at UC Berkeley, Spencer uses his background to provide education on topics including UTIs, urinary tract health, and non-antibiotic UTI treatments.

More about this author

There are many available methods for testing for urinary tract infections (UTIs). Some are more reliable than others. This is an overview of the current approaches to testing. A positive result in one of these tests may be a good indication that an infection is present. However, it is very important to note that when used individually these tests are not definitive evidence that an infection is not present.

UTI test strips are a quick, easy, and cheap way to test for a UTI at home. If both testing pads are coming up positive, it is very likely a UTI is present.

Standard UTI test strips, or urine dipsticks, are cheap, quick methods for indicating if a UTI might be present. To use them, they need to come into contact with the urine, typically during urination. Results are available within minutes. Urinary tract infection test strips often consist of 2 tests: a nitrite test and a leukocyte test. The nitrite test is used because gram-negative bacteria, like E. coli, convert nitrates into nitrites in the urine. This process doesn’t happen naturally, so the presence of nitrites may indicate a bacterial infection. These nitrites, however, are not produced by all bacteria. The leukocyte test looks for the presence of an enzyme called leukocyte esterase, which is produced by the leukocytes. Leukocytes are white blood cells, which are normally only present in the urine during an infection.

When both of these are positive, studies have shown they have a predictive power of up to 92% (1). UTI test strips may only be useful in identifying some UTIs, like those caused by E. coli, and a negative test does not rule out infection. If one is experiencing UTI symptoms but tests negative using an at-home UTI test strip, it is recommended they see a medical professional for a urine culture.

In addition, the quality of these tests matters. The tests should be individually wrapped, as to not be exposed to the air, and they should be stored in a manner in which they are not exposed to high temperatures.

Urine cultures have typically been considered the “gold standard” for diagnosing a UTI. Cultures are more expensive and take longer to get results (at least 48 hours) but may be more useful in determining if a UTI is present. While they can be administered at home, urine cultures are most commonly used in healthcare or lab test settings.

To perform a culture, urine is collected (ideally, midstream urine) and provided to a lab. At the lab, a sample of the urine is placed on a petri dish. Inside the petri dish is a substance called agar that contains nutrients that act as food for the bacteria. The petri dish is then set aside for up to 48 hours. Over the course of a few days, if bacteria are present in the urine sample, they grow into “colonies”. If there are no or very few colonies present, the test is considered negative. If there are a significant number of colonies, the test is considered positive and further tests are performed to identify the type of bacteria the colonies are composed of.

While urine cultures are very useful for determining the presence of a UTI, they can be inaccurate. A 2017 article published in the journal of Clinical Microbiology and Infection found that about 1 in 5 women who tested negative for a UTI in a urine culture had E. coli in the urine sample when tested using a more sensitive technique (quantitative PCR — see below) (2).

There are several reasons why urine cultures may not be accurate:

The bacteria causing the infection may not be in the urine. Some infections are caused by bacteria that are embedded within or on the surface of the bladder wall, protected by “biofilm”. The bladder cells cry for help, causing an immune response and UTI symptoms, but since the bacteria are protected by biofilm, they cannot be found by the white blood cells coming to help. Since these bacteria are not free-floating in the urine, they won’t be present in the urine sample. You can read more about biofilm and how it can turn an acute UTI into chronic UTIs. While white blood cells are present in the urine sample and suggestive of an infection, they may be ignored in cultures.

Not all pathogens can be cultured. In fact, the vast majority of bacteria cannot be detected this way. While E. coli can be grown in the agar described above, many bacteria simply can’t, or they are sensitive and do not survive the process. If the bacteria can’t grow or are dead, the colonies won’t develop, and the test will be negative.

The sample may be considered contaminated. When there are several types of bacteria present, these results can be thrown out as “contaminated”. The thinking is that the sample was contaminated by bacteria from the skin or something else. However, we now know that the urine is not sterile, and many types of microorganisms live naturally in urine. It is possible that the presence of these microbes could cause a result that would be thrown out as contaminated, even though it is positive. In addition, particularly in more complicated postmenopausal recurring UTIs, multiple types of bacteria can actually be causing the UTI at the same time.

EQUC is a type of urine culture that can detect a wider variety of microorganisms that otherwise could not be grown. These cultures use different culture mediums and conditions to optimize for the growth of more bacteria. Some reports indicate that bacteria was present in up to 90% of urine samples that would have otherwise been reported negative via a standard urine culture (3). While not yet widespread, EQUC provides richer information on what bacteria might be present in the urine sample.

Another new area of pathogen detection in the urine uses DNA or RNA sequencing techniques to identify hard to find or unusual microbes that might be causing, or be involved in, a UTI. Techniques like PCR or 16s rRNA tools use genetic markers to try and identify the presence of specific types of bacteria or other microbes. In addition, there are techniques that can even attempt to analyze all DNA present in a sample, determining all microbes that are currently known and categorized. These techniques are more sensitive, can detect much lower levels of microbes, and do not rely on the ability to culture a particular microbe.

While this testing is still in its infancy and the usefulness not completely understood, there are a growing number of case studies in which chronic UTI symptoms were caused by an unusual type of pathogen that otherwise was unable to be detected. It is important to note that pathogens that cause UTIs are not always bacteria but can be fungi (like yeast) or even parasites. These tests are currently expensive, but if other testing is negative, yet UTI symptoms persist, they are an option for cases than cannot be resolved. When conducting these tests, because they are very sensitive, it is extremely important that urine samples are collected carefully and are not contaminated.

UTI tests range in ease and expense, but each can be useful depending on the situation. The tests are presented in this article in the order in which they should be considered, with each test being more expensive and more difficult to obtain and interpret. Again, it is important to note that none are definitive and, at least alone, are not clear evidence that an infection is not present.

1. Little P, Turner S, Rumsby K, Jones R, Warner G, Moore M, Lowes JA, Smith H, Hawke C, Leydon G, Mullee M. Validating the prediction of lower urinary tract infection in primary care: sensitivity and specificity of urinary dipsticks and clinical scores in women. British Journal of General Practice. 2010 Jul; 60(576):495-500.

2. Heytens S, De Sutter A, Coorevits L, Cools P, Boelens J, Van Simaey L, Christiaens T, Vaneechoutte M, Claeys G. Women with symptoms of a urinary tract infection but a negative urine culture: PCR-based quantification of Escherichia coli suggests infection in most cases. Clinical Microbiology and Infection. 2017 Sep; 23(9):647-652.

3. Price TK, Dune T, Hilt EE, Thomas-White KJ, Kliethermes S, Brincat C, Brubaker L, Wolfe AJ, Mueller ER, Schreckenberger PC. The Clinical Urine Culture: Enhanced Techniques Improve Detection of Clinically Relevant Microorganisms. J Clin Microbiol. 2016 May; 54(5):1216-22.