An autoclave operator validates a cycle at 121°C for 15 minutes using a biological indicator (BI). The BI contains spores of Geobacillus stearothermophilus with a D-value of 1.5 minutes. How many D-values are achieved in this cycle, and what is the theoretical sterility assurance level (SAL) concept this demonstrates?
- A 6 D-values; SAL 10⁻⁶ — standard sterility assurance level for medical devices ✓
- B 10 D-values; SAL 10⁻¹⁰ ensuring a 10-log reduction in spore count
- C 10 D-values; achieving SAL 10⁻⁶ requires only 6 D-values so this is a safety margin
- D 15 D-values; SAL 10⁻¹⁵ used for implantable devices
Correct answer: A. 6 D-values; SAL 10⁻⁶ — standard sterility assurance level for medical devices
Explanation
D-value is the time to reduce microbial population by 90% (1 log₁₀). At 121°C with D = 1.5 min, 15 minutes achieves 10 D-values (15÷1.5 = 10). The standard SAL for medical devices is 10⁻⁶ (probability of one organism surviving in 10⁶ items), which requires 6 D-values assuming initial bioburden of 10⁶. The additional D-values (up to 10 in a 15-minute cycle) provide the safety margin called 'overkill'. The BI test with G. stearothermophilus (D₁₂₁ = 1.5-2 min) is the gold standard biological monitor for steam sterilisation.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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