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Medical Microbiology

Optochin Susceptibility Test

Optochin Susceptibility Test is a Medical Microbiology laboratory test reference in the LabPedia Medical Laboratory Library. Disk test distinguishing Streptococcus pneumoniae (optochin-susceptible) from viridans streptococci (usually resistant).

Also known asP disk; Ethylhydrocupreine test

Overview

Optochin (ethylhydrocupreine) disks inhibit S. pneumoniae growth, producing a zone of inhibition used as a presumptive identification test.

Clinical Importance

Identify S. pneumoniae among alpha-hemolytic streptococci

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

A blood agar plate is streaked and an optochin disk applied. After CO₂ incubation, zone diameter is measured against CLSI/institutional interpretive criteria (≥14 mm commonly used for susceptibility with 6 mm disks; verify current method insert/SOP).

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.

Zone interpretive criteria are method-specific; use the laboratory SOP / CLSI guidance adopted locally. Bile solubility may confirm discrepant isolates.

Interpretation

Common Causes of Increased Results

  • Zone meeting susceptible criteria; presumptive S. pneumoniae

Common Causes of Decreased Results

  • No significant zone; consistent with viridans group streptococci (confirm as needed)

Clinical Significance

  • Alpha-hemolytic streptococcal colony workup from respiratory and sterile sites

Factors Affecting Results

Pre-analytical

  • Wrong atmosphere (needs CO₂ for reliable zones)

Analytical

  • Measuring zones incorrectly
  • Optochin-resistant pneumococci (uncommon) requiring bile solubility/MALDI correlation

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

  • Simple classical pneumococcal screen

Limitations

  • Rare resistant pneumococci
  • Not a susceptibility result for therapy

Clinical Pearls

Optochin is an identification disk; do not paste it into the patient’s AST report as a drug result.

Related Laboratory Tests

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 Optochin Susceptibility Test laboratory test Medical Laboratory Library reference in Medical Laboratory Science education.

What is the Optochin Susceptibility Test laboratory test reference used for?

This Medical Laboratory Library page summarizes the clinical importance of Optochin Susceptibility Test. Review the Clinical Importance section for why the test is ordered in laboratory medicine practice.

Where can I find Optochin Susceptibility Test specimen requirements?

See the Specimen Requirements section on this page for specimen information related to Optochin Susceptibility Test. Confirm collection details against your laboratory protocol.

Where are Optochin Susceptibility 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 Optochin Susceptibility 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 Optochin Susceptibility 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 Optochin Susceptibility 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 Optochin Susceptibility 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 Optochin Susceptibility Test?

See the Clinical Pearls section for concise laboratory practice notes associated with Optochin Susceptibility Test. Apply pearls alongside institutional protocols.

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