What does SD mean on an antibiotic sensitivity test?

• A. Standard dose
• B. Susceptibility dose
• C. Standard deviation
• D. Variable dose

Answer: (D)

SD on an antibiotic sensitivity test signals susceptible-dose dependent interpretation: the organism’s susceptibility depends on achieving higher drug exposure. For certain antibiotic–organism combinations, standard dosing may not classify the organism as fully susceptible, but higher doses can reach the concentrations needed to inhibit growth. This concept ties directly to PK/PD principles like time above MIC or Cmax/MIC, depending on the drug class. So, the idea is that dose, not a fixed standard, influences whether the drug will be effective. The phrase “variable dose” isn’t a standard interpretation on sensitivity reports, whereas recognizing dose-dependent susceptibility helps guide whether a higher-dose regimen could be used safely. If you see SD, check the lab’s legend, but the key concept is that susceptibility can depend on the dose.