The practical value of Urea / INDOLE Test is not the plate or broth alone; it is the reproducibility of the susceptibility endpoint produced from it. The discussion below connects its documented role with preparation variables, controls, interpretation, and the limits of inference. Catalogue 88024 is discussed as a laboratory measurement component; implementation should follow the laboratory SOP and validated reference method.
Analytical scope and method role
Rapid test for the detection of urease and the evaluation of indole production used for the differentiation and identification of bacterial strains. This test is intended as an aid in the diagnosis, requiring further tests to complete the diagnostic results.
Method principle
The enzyme urease catalyzes the hydrolysis of urea to carbon dioxide, water and ammonia with increase in pH and a color change of the phenol red indicator to red-fuchsia. Bacteria that produce the enzyme tryptophanase are able to degrade the amino acid tryptophan into pyruvic acid, ammonium and indole. The indole produced reacts with p-dimethylaminobenzaldehyde of the Kovac’s (indole) reagent to form a red-violet compound. Enteric pathogens such as Salmonellae and Shigellae are urease negative whilst organisms such as Proteus, Morganella, Klebsiella spp and some species of Citrobacter are strongly urease positive. The production of indole from tryptophan is a characteristic absent in Salmonella but present in E. coli, Morganella and some species of Klebsiella, Aerobacter and Citrobacter.
Preparation and execution: controlled method points
Procedure: 1. Take the number of Urea/Indole Test tubes needed from the fridge and allow them to reach room temperature. 2. Add 0.3 mL of physiological solution to each tube. 3. Inoculate heavily with the test organism(*) from a fresh overnight pure culture. 4. Incubate at 35 ± 2°C for up to 24 hours(**). 5. Examine tubes at intervals of 1, 4 and 24 hours for broth colour change. 6. Add 2-3 drops of Kovac's Reagent to perform the Indole Test. * Microbial suspension equivalent to 0.5 McFarland turbidity standard (which corresponds to 1-2 x 108 CFU/ml for E. coli). ** Prolonged incubation may result in a false-positive test due to hydrolysis of proteins in the medium. © Liofilchem - Urea/Indole Test - Rev.3 / 16.09.2024 Page 2 of 11 Note: Positive and negative controls should be run simultaneously with the organism to be tested (see
Translate the method into controlled worksheet fields: catalogue and lot, preparation mass or volume, water quality, pH where applicable, heating or sterilization, supplements, dispense volume, preparation date, expiry assignment, incubation atmosphere and temperature, reading window, equipment, and analyst. Record departures as deviations instead of silently normalizing them.
Quality control: what the control result protects
organisms. REAGENTS • A tube of Urea/Indole Test contains desiccated biochemical substrates and nutritive components such as urea, phenol red and tryptone, which includes tryptophane. • Kovac’s Reagent consists of 5% p-dimethylaminobenzaldehyde dissolved in a solution of 25% hydrochloric acid and 75% isobutyl alcohol. SPECIMEN Collect specimens in sterile containers or with sterile swabs and transport to the laboratory. Process each specimen using procedures appropriate for that sample. This product is recommended for use only with pure cultures.
Challenge both sides of performance. Include a target reaction demonstrating recovery and, when relevant, a non-target or alternate reaction challenging inhibition or differentiation. Record expected and observed responses, acceptance criteria, strain and passage, lot, incubation, and disposition. A failed control blocks result interpretation until the cause is resolved.
Interpretation and confirmation pathway
A positive urease reaction is indicated by a colour change to red-fuchsia. The
Record the raw observation before the conclusion: morphology or colour, growth pattern, endpoint, reaction intensity, reading time, and control response. Presumptive reactions remain presumptive. Route atypical, weak, mixed, or delayed reactions through the confirmation step in the current method.
Storage, stability, and pre-use inspection
2-8°C in its original packaging. Keep away from sources of heat and avoid excessive changes of temperature. Use until the expiry date indicated on the label. Eliminate without using if there are signs of deterioration.
Receipt and storage records should reconstruct temperature exposure, lot rotation, expiry, container integrity, and time outside controlled storage. Inspect colour, clarity, fill volume, gel integrity, contamination, dehydration, and precipitate before use.
Limitations and boundaries of use
Due to nutritional variation, some strains may result in poor growth or fail to grow on this medium providing false negative results. As the urea test is based on demonstration of alkalinity, it is not specific for urease. The hydrolysis of proteins by contaminating organisms in the medium may raise the pH to alkalinity resulting in false positive reactions. Allow at least 30 seconds for the color to develop before considering the indole test negative. Pigmented organisms may yield ambiguous results.
Applicability depends on matrix, target organism, interfering flora, inoculum, validated method, and regulatory setting. Do not transfer conditions or acceptance criteria from a different matrix without verification.
Troubleshooting from evidence, not guesswork
- Poor recovery: check preparation, pH, overheating, supplement addition, inoculum, atmosphere, storage, and control performance.
- Loss of selectivity: review formulation, sterilization, expiry, inoculum burden, enrichment sequence, and incubation time.
- Weak or atypical reaction: confirm reading time and atmosphere, compare controls, document mixed growth, and follow confirmation requirements.
- Run-to-run drift: trend lot, analyst, equipment, water, preparation batch, controls, and environmental conditions.
External method context
The product should be understood as one measurement component in an antimicrobial susceptibility method. The analytical chain includes isolate purity, standardized inoculum, medium composition, antimicrobial concentration, incubation atmosphere and time, endpoint reading, and the interpretive table selected by the laboratory. A shift in any upstream variable can move an MIC or zone without a change in organism biology, which is why QC distributions and trend review are more informative than a single pass/fail control.
External laboratory-quality guidance also supports a lifecycle view: verify the method when introduced, bridge new lots against an accepted lot, trend control results, investigate nonconformities, and retain enough raw observations to reconstruct the decision. This is the practical difference between reproducing a preparation instruction and demonstrating that the resulting measurement is fit for its intended use.
FDA BAM Chapter 5: Salmonella
The current FDA BAM Salmonella workflow separates pre-enrichment, selective enrichment, selective plating, and confirmation. That sequence is scientifically important: enrichment increases target representation in a mixed matrix, selective conditions manage competing flora, and confirmation prevents a colour or colony morphology from being treated as identity. A laboratory transferring the method should verify recovery and selectivity on its own matrix classes rather than assuming that a medium performs identically across foods, water, and environmental samples. Review the authoritative source.
EUCAST disk diffusion and quality control
EUCAST requires standardized methodology, current breakpoint tables, and routine quality control; the laboratory remains responsible for the final susceptibility report. For broth dilution, inoculum density, cation content, pH, incubation atmosphere, endpoint definition, and two-fold dilution design influence MIC reproducibility. Breakpoints are interpretive rules applied after a technically valid MIC or zone measurement; they are not properties of the product alone. Review the authoritative source.
WHO Laboratory Quality Management System handbook
WHO laboratory quality guidance treats document control, equipment, purchasing and inventory, process control, records, occurrence management, and continual improvement as connected parts of reliable testing. Review the authoritative source.
Technical references
- Manufacturer product documentation for catalogue 88024.
- FDA BAM Chapter 5: Salmonella (access checked 2026-07-20).
- EUCAST disk diffusion and quality control (access checked 2026-07-20).
- WHO Laboratory Quality Management System handbook (access checked 2026-07-20).
Bench-use checklist
- Verify product identity, catalogue number, lot, expiry, and storage history.
- Document preparation and incubation exactly as controlled in the SOP.
- Define controls and acceptance criteria before testing.
- Record raw observations, control response, reading time, and confirmation decision.
- Escalate failed controls, atypical reactions, and departures through the quality system.