Overview
The indole test detects indole formed from tryptophan; Kovacs or Ehrlich reagent yields a red/pink ring or spot color when indole is present.
Clinical Importance
Support rapid E. coli presumptive identification
Contribute to Enterobacterales biochemical profiles
Specimen Requirements
- Specimen: pure culture isolate (colony) from primary clinical culture
- Source: organism recovered on appropriate agar after incubation
- Handling: test fresh colonies within the timeframe validated by the method (typically 18–24 h cultures for many spot tests)
- Controls: run positive and negative control organisms per SOP/QC schedule
Analytical Method
Spot indole applies reagent to growth from tryptophan-containing media (often blood agar). Tube methods grow organisms in tryptophan broth before reagent addition. Color development indicates indole positivity.
Instruments Used
Automated analyzers
- Automated identification systems that incorporate equivalent biochemical reactions (VITEK 2, MicroScan, Phoenix)
- MALDI-TOF MS (often used instead of classical spot tests for species ID)
Manual methods
- Manual tube and spot biochemical tests with visual endpoint reading
Reference Values
Microbiology results are interpreted against organism identification, specimen quality, clinical context, and current CLSI and/or EUCAST interpretive criteria (including CLSI M100 for susceptibility where adopted). Reporting conventions and critical-notification lists are institution-defined; verify against the performing laboratory SOP and current guideline editions.
Qualitative: positive or negative. Interpret only as part of a full identification profile.
Interpretation
Common Causes of Increased Results
- Positive; supports E. coli and other indole-positive organisms in context
Common Causes of Decreased Results
- Negative; directs algorithms away from classical indole-positive E. coli patterns
Clinical Significance
- Enteric Gram-negative identification
- Presumptive lactose-fermenting colony workups
Factors Affecting Results
Pre-analytical
- Testing from media lacking tryptophan
Analytical
- Weak reactions misread
- Reagent deterioration
Post-analytical
- Reporting colonizers as pathogens without clinical/site correlation
- Wrong interpretive criteria (CLSI vs EUCAST; outdated breakpoints)
- Failure to notify critical positive cultures per institutional policy
Advantages and Limitations
Advantages
- Rapid spot format widely available
Limitations
- Not unique to E. coli
- Superseded by MALDI-TOF for final species ID in many labs
Clinical Pearls
Indole-positive and lactose-positive on MacConkey is the classic E. coli hint; still confirm before you stake the report on two colors.
References
Authoritative textbooks, guidelines, and reviews supporting this laboratory test reference. Verify reference intervals, critical limits, and method details against institutional protocols and current guideline editions.
Textbooks
- Carroll KC, Pfaller MA, Landry ML, et al., eds. Manual of Clinical Microbiology. ASM Press (verify current edition).
- Forbes BA, Sahm DF, Weissfeld AS. Bailey & Scott's Diagnostic Microbiology. 15th ed. Elsevier; 2020.
- Mahon CR, Lehman DC, Manuselis G. Textbook of Diagnostic Microbiology. 6th ed. Elsevier; 2018.
- Carroll KC, Hobden JA, Miller S, et al. Jawetz, Melnick, & Adelberg's Medical Microbiology. 28th ed. McGraw-Hill; 2019.
- Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology. 9th ed. Elsevier; 2021.
CLSI and Professional Guidelines
- Clinical and Laboratory Standards Institute. Performance Standards for Antimicrobial Susceptibility Testing. CLSI document M100 (verify current edition and institutional adoption).
- Centers for Disease Control and Prevention / NIH. Biosafety in Microbiological and Biomedical Laboratories (BMBL). Current edition (verify institutional adoption).
WHO Publications
- World Health Organization. Laboratory and diagnostic guidance relevant to clinical microbiology practice (verify current WHO publication for the method).
Frequently Asked Questions
Common questions about using this Indole Test laboratory test Medical Laboratory Library reference in Medical Laboratory Science education.
What is the Indole Test laboratory test reference used for?
This Medical Laboratory Library page summarizes the clinical importance of Indole Test. Review the Clinical Importance section for why the test is ordered in laboratory medicine practice.
Where can I find Indole Test specimen requirements?
See the Specimen Requirements section on this page for specimen information related to Indole Test. Confirm collection details against your laboratory protocol.
Where are Indole Test reference values listed?
See the Reference Values section on this Laboratory Library page. Reference ranges can vary by method and population, so verify intervals with your laboratory.
How do I interpret Indole Test results?
Use the Interpretation section for laboratory interpretation framing, including increased and decreased result patterns when provided. Do not treat educational text as a patient-specific diagnosis.
What analytical method is used for Indole Test?
See the Analytical Method section for principle and method notes. Method details may differ by instrument and institutional SOP.
Are related laboratory tests linked from Indole Test?
Yes. The Related Laboratory Tests section lists connected references for continued lookup in the Medical Laboratory Library.
How can I continue learning after reading Indole Test?
Use Continue learning links on this page for related Academy lessons, medical laboratory quizzes, or laboratory case studies when available for this Medical Microbiology topic.
What are the clinical pearls for Indole Test?
See the Clinical Pearls section for concise laboratory practice notes associated with Indole Test. Apply pearls alongside institutional protocols.
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