LD50 refers to the dose lethal to what proportion of test units?

• A. 100%
• B. 50%
• C. 10%
• D. 25%

Answer: (B)

LD50 is the median lethal dose—the amount of a substance required to kill 50% of a defined test population. This measure captures the potency of a toxicant: a smaller LD50 means the substance is more lethal because a lower dose causes death in half the individuals. To determine it, data from groups exposed to different doses are used to construct a dose–response curve and estimate the dose at which 50% die (often using probit or related analyses). Other lethal-dose terms exist for different killed fractions, like LD100 (all die) or LD10/LD25 (10% or 25% die), but LD50 specifically refers to the 50% point.