Urine dipstick

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Reference chart

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.[1] 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.

Contents

Blood

  • 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

Protein

  • Positive = proteinuria
  • Often quantified and denoted as +, ++ , +++

Glucose

  • 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.[2] False negatives can result from presence such as ketones and ascorbic acid.

pH

Bilirubin

Nitrite

  • 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.[3]
  • 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.).

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

References

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