Overview
The D-test places erythromycin and clindamycin disks in proximity; flattening of the clindamycin zone (D-shape) indicates inducible resistance.
Clinical Importance
Prevent false clindamycin-susceptible reports when inducible MLSᴮ resistance is present
Meet CLSI/EUCAST supplemental testing expectations for relevant isolates
Specimen Requirements
- Specimen: pure culture isolate of the organism to be tested
- Inoculum: prepare to the McFarland standard specified by the method (commonly 0.5 McFarland for many disk/MIC methods)
- Media: use CLSI/EUCAST-recommended medium (e.g., Mueller-Hinton agar ± blood for fastidious organisms)
- Controls: include QC organisms on the validated schedule
Analytical Method
On a disk diffusion plate, erythromycin and clindamycin disks are placed at a standardized distance. After incubation, a D-shaped blunting of the clindamycin zone facing erythromycin indicates inducible clindamycin resistance; clindamycin is then reported resistant (per current guidance).
Instruments Used
Automated analyzers
- Some automated systems include inducible clindamycin detection algorithms; confirm performance characteristics locally
Manual methods
- Manual D-test on Mueller-Hinton (± blood) agar per CLSI placement rules
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: D-test positive (inducible resistance) or negative. Follow current CLSI M100 reporting rules for staphylococci/streptococci.
Interpretation
Common Causes of Increased Results
- D-shaped zone; report clindamycin resistant despite a large zone on the side opposite erythromycin
Common Causes of Decreased Results
- No blunting; clindamycin may be reported susceptible if zone/MIC criteria are met
Clinical Significance
- Staphylococcal and streptococcal AST supplemental testing
- Correction of misleading automated clindamycin-susceptible calls
Factors Affecting Results
Pre-analytical
- Incorrect disk spacing
Analytical
- Missing subtle D-zones
- Wrong medium/atmosphere
Post-analytical
- Releasing clindamycin-susceptible results without required D-test
Advantages and Limitations
Advantages
- Simple phenotypic detection of inducible resistance
Limitations
- Does not detect all resistance mechanisms
- Requires disciplined reading
Clinical Pearls
If erythromycin is R and clindamycin looks S, look for the D before you bless clindamycin.
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).
- Clinical and Laboratory Standards Institute. Performance Standards for Antimicrobial Susceptibility Testing. CLSI document M100 (verify current edition and institutional adoption).
- Clinical and Laboratory Standards Institute. Performance Standards for Antimicrobial Disk Susceptibility Tests. CLSI document M02 (verify current edition).
- Clinical and Laboratory Standards Institute. Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically. CLSI document M07 (verify current edition).
- European Committee on Antimicrobial Susceptibility Testing (EUCAST). Breakpoint tables and documents (verify current version and 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 D-test laboratory test Medical Laboratory Library reference in Medical Laboratory Science education.
What is the D-test laboratory test reference used for?
This Medical Laboratory Library page summarizes the clinical importance of D-test. Review the Clinical Importance section for why the test is ordered in laboratory medicine practice.
Where can I find D-test specimen requirements?
See the Specimen Requirements section on this page for specimen information related to D-test. Confirm collection details against your laboratory protocol.
Where are D-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 D-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 D-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 D-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 D-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 D-test?
See the Clinical Pearls section for concise laboratory practice notes associated with D-test. Apply pearls alongside institutional protocols.
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