What are the D‑zone variations observed in clindamycin susceptibility testing for skin infections?

Medical Advisory BoardAll articles are reviewed for accuracy by our Medical Advisory Board
Educational purpose only • Exercise caution as content is pending human review
Article Review Status
Submitted
Under Review
Approved

Last updated: September 18, 2026 • View editorial policy

Have a follow-up question?

Our Medical A.I. is used by practicing medical doctors at top research institutions around the world. Ask any follow up question and get world-class guideline-backed answers instantly.

Ask Question

D-Zone Test Variations in Clindamycin Susceptibility Testing

The D-zone test should be performed at 15 mm edge-to-edge distance between erythromycin and clindamycin disks, rather than the standard 21-25 mm distance, to accurately detect inducible clindamycin resistance in skin infections caused by Staphylococcus aureus and streptococci.

Understanding D-Zone Testing

The D-zone test (disk approximation test) is a critical screening method for detecting inducible clindamycin resistance (iMLSB phenotype) in erythromycin-resistant, clindamycin-susceptible isolates. This resistance mechanism is mediated by erm genes (primarily ermC in S. aureus) that encode ribosomal methylation, which can be induced during therapy, leading to treatment failure 1.

Key Distance Variations

Standard vs. Optimized Distance

The most clinically significant variation involves the inter-disk distance:

  • Standard distance: 21 ± 1 mm or 25 mm edge-to-edge (per manufacturer recommendations)
  • Optimized distance: 15 mm edge-to-edge

Research demonstrates that using 15 mm distance detects significantly more inducible resistance cases 2, 3:

  • 84.3% increase in positive D-tests among S. aureus
  • 43.8% increase among group B streptococci
  • 36.9% of isolates initially negative at 25 mm tested positive at 15 mm

When the standard 25 mm distance was used, 43% of isolates gave false-negative results that were subsequently identified as iMLSB-positive at 15 mm 2.

Clinical Implications for Skin Infections

Why This Matters

For skin and soft tissue infections, clindamycin is a recommended empirical option for CA-MRSA coverage 4, 5. However, the clinical significance of inducible clindamycin resistance is particularly important for serious infections 5. While the drug may still be effective for mild infections despite inducible resistance, its presence should preclude clindamycin use for more severe infections.

Prevalence Data

Studies show concerning rates of inducible resistance:

  • 23.5% overall among erythromycin-resistant Staphylococcal species 6
  • 87% of iMLSB isolates were methicillin-resistant 6
  • 12-23% of MRSA isolates demonstrate iMLSB phenotype 5, 7

Testing Methodology Variations

Phenotypic Patterns Observed

The D-zone test reveals distinct phenotypes 6, 8:

  1. D phenotype (18.13%): Classic flattening of clindamycin zone adjacent to erythromycin disk
  2. D+ phenotype (5.34%): More pronounced blunting with antagonism
  3. cMLSB phenotype: Constitutive resistance (both drugs resistant)
  4. MS phenotype: Efflux-mediated (erythromycin-resistant, clindamycin-susceptible, no induction)

Testing Medium Considerations

The D-test can be performed on:

  • Mueller-Hinton agar (standard reference method) 9
  • Sheep blood agar (when using automated systems like VITEK 2) 9

Both media correlate well with PCR detection of erm genes 9.

Practical Testing Algorithm

For erythromycin-resistant, clindamycin-susceptible isolates from skin infections:

  1. Place erythromycin (15 μg) and clindamycin (2 μg) disks 15 mm apart (edge-to-edge) on Mueller-Hinton agar
  2. Incubate 18-24 hours at 35°C
  3. Interpret results:
    • Positive D-test: Flattening of clindamycin zone = inducible resistance → avoid clindamycin
    • Negative D-test: Circular clindamycin zone = true susceptibility → clindamycin appropriate

Common Pitfalls

Critical Errors to Avoid

  1. Using 25 mm distance: Misses 36.9% of inducible resistance cases 2
  2. Not performing D-test at all: Standard automated susceptibility testing reports clindamycin as "susceptible" without detecting inducibility 9
  3. Ignoring positive D-test for "mild" infections: While controversial, treatment failures can occur 5
  4. Testing only MRSA: GBS and other streptococci also demonstrate inducible resistance 2

Special Populations

For hospitalized children with complicated SSTI: Clindamycin is recommended only if local resistance rates are <10%, with transition to oral therapy if the strain is susceptible 4, 5. This underscores the importance of accurate D-zone testing.

Genotypic Correlation

The 15 mm D-test correlates well with molecular detection 9, 7:

  • ermC gene: Most common (30.5% of isolates), causes both constitutive and inducible resistance
  • msrA gene: Causes MS phenotype (9.4%), clindamycin remains effective
  • ermA/ermB: Rare in S. aureus skin isolates

The D-test at 15 mm distance is essential for routine antimicrobial susceptibility testing to prevent therapeutic failures when treating skin infections with clindamycin, particularly given the high prevalence of methicillin-resistant isolates with inducible resistance 6.

References

Related Questions

Is clindamycin appropriate for treating a skin and soft‑tissue infection with an abscess at the [LOCATION]?
What is the recommended clindamycin dose and schedule for treating cellulitis in an adult with normal liver and renal function and no known lincosamide allergy?
What is the appropriate clindamycin regimen for treating a furuncle, including dosage, duration, and contraindication considerations?
Can clindamycin vaginal cream and clindamycin vaginal suppository be used simultaneously?
Is a clindamycin dose of 300 mg three times daily appropriate for an adult with normal liver function?
What natural supplements can an adult with confirmed Helicobacter pylori infection use as adjuncts to standard antibiotic therapy, including recommended agents, dosages, duration, and safety considerations?
Can Adderall (mixed amphetamine salts) be taken together with Imodium (loperamide)?
Could diabetes insipidus (DI) be responsible for my severe watery diarrhea and near‑syncope after ingesting an entire bottle of magnesium citrate?
When and how should continuous veno‑venous hemodialysis (CVVHD) be used in critically ill patients with acute kidney injury, including indications, prescription settings, and anticoagulation recommendations?
What are the most common causes of infertility in an otherwise healthy 31‑year‑old woman with a two‑year history of inability to conceive, given that hysterosalpingogram (HSG), pelvic ultrasound, routine blood work, urine testing, and partner sperm analysis are all normal?
What is the appropriate management for low‑normal potassium measured two days after a magnesium‑citrate–induced osmotic diarrhea?

Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.