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Reader task: a lab or clinic staff member or student looking at a urea tube or plate and deciding what a pink or yellow result means. This page is general product and laboratory-reference information, not medical advice.
Short answer
A urease test is positive when a urea medium turns bright pink (fuchsia) and negative when it stays yellowish. The colour change comes from ammonia released by urea hydrolysis, which makes the medium alkaline and turns the pH indicator pink. Because the test reads alkalinity, a pink result is strong evidence of urease activity but not proof by itself.
Definition and scope
The urease test identifies organisms that hydrolyze urea to ammonia and carbon dioxide. The American Society for Microbiology (ASM) protocol describes its main use as distinguishing urease-positive Proteeae from other Enterobacteriaceae (ASM Urease Test Protocol). It is one of several biochemical tests used alongside others when working out an isolate’s identity; for the wider workflow see identifying bacteria and, for a companion reading of an iron-sugar tube, triple sugar iron agar results.
What a positive and negative result look like
Christensen’s urea agar
Per ASM, a positive reaction shows as a bright pink (fuchsia) colour on the slant, which may extend into the butt. A negative tube stays yellowish. ASM gives an incubation window of 1 to 6 hours for Christensen’s medium.
Stuart’s urea broth
ASM describes Stuart’s urea broth as used mainly to differentiate Proteus species. A positive broth is bright pink throughout.
How the colour is produced
Remel Urea Agar (Thermo Fisher instructions for use) is a lightly buffered medium containing urea and phenol red. When organisms use urea, ammonia forms and the medium turns alkaline; phenol red changes from pale yellow to pink-red in an alkaline environment. The same document states that the gelatin peptone in that formulation promotes rapid growth of enteric gram-negative bacilli, and that dextrose serves to eliminate false-negative reactions and stimulates urease activity in organisms that hydrolyze urea slowly.
Read times by medium and organism group
| Medium or organism group | Reported timing | Source |
|---|---|---|
| Christensen’s urea agar | Incubation 1 to 6 hours | ASM Urease Test Protocol |
| Proteus spp. (rapid urease-positive) on Remel Urea Agar | Usually slant and butt pink-red in 2 to 6 hours; slant to be read within the first 2 to 6 hours | Remel Urea Agar instructions for use |
| Klebsiella and Enterobacter on Remel Urea Agar | Delayed reaction that may need 24 to 48 hours | Remel Urea Agar instructions for use |
| Remel Urea Agar examination schedule | Examine at 2, 6 and 24 hours, then daily up to 6 days | Remel Urea Agar instructions for use |
The practical point is that one fixed read time does not suit every organism: a rapid urease producer can be read within hours, while a slow one can look negative early and still turn pink later.
Why a pink slant is not always urease
The Remel instructions state that urea test media rely on demonstration of alkalinity and are not specific for detection of urease activity. Peptones in the medium may be hydrolyzed, releasing amino acid residues, raising the pH, and giving false-positive reactions. They note that a control test using the same medium without urea can help interpret questionable reactions.
Checklist: what the colour does and does not show
- Pink means the medium became alkaline. ASM attributes this to ammonia from urea hydrolysis.
- A weak or doubtful pink is the case where the Remel instructions describe a no-urea control as an aid.
- Yellow at an early read does not settle a slow reactor, because delayed reactions can take 24 to 48 hours.
- Remel’s limitations note that for rapid urease-positive organisms such as Proteus, the slant should be read within the first 2 to 6 hours, so a very late pink is interpreted against that limit.
Where urease sits among other biochemical tests
Urease is usually one result among many. The methyl red test belongs to the IMViC group of biochemical tests, and a Gram reaction from a Gram stain typically comes first. Related tube tests such as citrate utilisation and iron-sugar media answer different questions than urease, so their results are interpreted on their own pages rather than inferred from a urea tube.
Where it matters for labs and supply decisions
Differential culture media are regulated medical devices. Under 21 CFR 866.2320, a differential culture medium is a medium that cultivates and identifies pathogenic microorganisms, to which a specific biochemical component is added so that a visible change such as a colour change shows particular metabolic reactions; results help diagnose disease and provide epidemiological information. The regulation classifies the device as Class I (general controls) and exempt from premarket notification, subject to the limitations in 21 CFR 866.9 (eCFR). The practical consequence is that each lot’s labeling, intended use and quality-control organisms come from the manufacturer’s instructions for use, which is where the timing and limitations above are stated.
The Remel instructions list quality-control organisms for urea agar: Proteus mirabilis ATCC 12453 as positive, Escherichia coli ATCC 25922 as negative, and Cryptococcus neoformans ATCC 34877 as positive (24 to 48 hours at 25 to 30 degrees C). Laboratories keeping aseptic handling records can cross-reference aseptic technique.
Common misunderstandings
- Pink always means urease. Remel describes urea media as measuring alkalinity, so peptone breakdown can mimic a positive.
- Yellow after a few hours means negative. Delayed reactors such as Klebsiella and Enterobacter may need 24 to 48 hours on the Remel formulation.
- Stuart’s broth and Christensen’s agar are interchangeable. ASM assigns them different uses: Christensen’s for urease activity across a variety of microorganisms, Stuart’s mainly for differentiating Proteus species.
FAQ
What does a positive urease test look like?
Bright pink (fuchsia) on a Christensen’s agar slant, possibly extending into the butt, or bright pink throughout Stuart’s broth (ASM Urease Test Protocol).
What does a negative urease test look like?
A Christensen’s agar tube that stays yellowish (ASM). Delayed reactors can still turn pink later (Remel).
What is the urease test used for?
Identifying organisms that hydrolyze urea to ammonia and carbon dioxide, mainly to distinguish urease-positive Proteeae from other Enterobacteriaceae (ASM).
How do citrate test results, triple sugar iron agar and Kligler iron agar relate to urease?
They are separate biochemical tests that answer different questions. This page covers urease only; see triple sugar iron agar results and the IMViC-group methyl red page for those tests.
Sources
- American Society for Microbiology, Urease Test Protocol: https://asm.org/protocols/urease-test (checked 2026-10-30)
- Thermo Fisher Scientific (Remel), Urea Agar instructions for use, IFU65210, revised 3 February 2011: https://documents.thermofisher.com/TFS-Assets/LSG/manuals/IFU65210.pdf (checked 2026-10-30)
- 21 CFR 866.2320, Differential culture medium: https://www.ecfr.gov/current/title-21/part-866/section-866.2320 (checked 2026-10-30)
