Commercially available dipsticks can be used to test for various substances in the urine. Most commonly, several different reagents are attached as little squares to a slender plastic stick. Contact with the urine causes various biochemical and immunochemical reactions that result in a visual change. The corresponding colour change gives a rough indication on the amount of substance. A reference chart is provided for each test to semi-quantify the colour change. The exact selection of tests depends on the make and manufacturer. See also review of urinalysis: .
- Positive = haematuria (although false positives can result from urine contamination with another reducing agent, such as bleach from cleaning out a patient-supplied collection flask)
- Can be haemolysed or non-haemolysed
- Erythrocytes have peroxidase activity, allowing a colour change when mixed with hydrogen peroxide and a chromogen.
- As the name suggests, urine dipsticks should be reserved for analysing urine. It is inappropriate to use it for testing gastric contents which may contain non-blood perioxidases, invariably giving a false positive result.
- Positive = proteinuria
- Often quantified and denoted as +, ++ , +++
- Accurate quantification best done as protein-creatine ratio
- Often used as a screening test for impaired glucose tolerance or diabetes
- Several different enzymes can be linked to chemical colour changes. A common choice is the enzyme glucose oxidase which converts glucose to lactone and hydrogen peroxide. In the presence of a peroxidase, the hydrogen peroxide oxidises a dye agent, causing a visible colour change.
- False results depend on type of enzyme system. False negatives can result from presence such as ketones and ascorbic acid.
- For detection of ketone bodies, although the assay typically only detects acetoacetate and not acetone or β-hydroxybutrate.
- Detects conjugated bilirubin.
- Certain bacteria are able to convert nitrate to nitrite. Importantly, eukaryotic cells cannot. Chemically, nitrites are detected by the Griess reaction which produces a pink pigment if positive.
- A positive results is highly suggestive of a UTI, but a negative result does not exclude one.
- Organisms that do not produce nitrite include Actinobacter spp., Enterococci and Staphlyococcus saprophyticus.
- False negatives may result as some organisms are able to further metabolise nitrite to ammonium. A diet low in vegetables, reduces the amount of excreted nitrates, causing less substrate for reduction of nitrate to nitrite.
- Various dietary compounds can also interfere with this step, e.g. oxidising agents like vitamin C.
Leucocyte esterase activity
- Detects the presence of leucocytes, usually from pus in the urine, pyuria.
- Commonly raised in UTI, but may be due to sterile pyuria (e.g. appendicitis, chemical cystitis, etc.).
- A specific dipstick is required. See pregnancy test.
Finally, it's a good idea to wash your hands after checking urine dipsticks, as countless urine soaked sticks will have been held against the chart on the container!
Sensitivity in Detecting UTI
- ↑ http://w3.whosea.org/bct/micro/22.htm
- ↑ Simerville JA, Maxted WC, Pahira JJ. Urinalysis: a comprehensive review. American family physician. 2005 Mar 15; 71(6):1153-62.
- ↑ http://www.nlm.nih.gov/medlineplus/ency/article/003581.htm
- ↑ St John A, Boyd JC, Lowes AJ, Price CP. The use of urinary dipstick tests to exclude urinary tract infection: a systematic review of the literature. American journal of clinical pathology. 2006 Sep; 126(3):428-36.(Link to article – subscription may be required.)