How many laboratories are necessary? Analytical assessment of precision and reproducibility in disinfectant efficacy testing for quantifying the impact of laboratory number on result reliability.

BACKGROUND: Reliable assessment of the efficacy of chemical disinfection procedures requires robust experimental precision and reproducibility. In order to obtain a certificate from the Association for Applied Hygiene (VAH) proving the efficacy of a disinfectant procedure, test results from two independent accredited laboratories must be submitted for evaluation. In a previous study, Monte-Carlo simulations based on VAH ring trial data demonstrated that this two-laboratory stipulation significantly reduces the probability of false-positive efficacy statements. The present study extends that work by developing an analytical solution to quantify the influence of intra- and interlaboratory variability on efficacy assessment outcomes.

METHODS: The analysis is based on data from four VAH ring trials covering bactericidal, yeasticidal, and mycobactericidal activities as tested according to EN 13727, EN 13624, EN 17387 and EN 14563. Analytical probability models were derived to calculate the likelihood that one, two, three, or four laboratories independently classify a disinfectant as effective under defined test conditions. The results were compared across different efficacy ranges and interpreted in relation to the existing two-laboratory validation framework.

RESULTS: The analytical solution confirms that including a second laboratory significantly enhances the robustness of efficacy classification by reducing the probability of false-positive outcomes in the intermediate efficacy range by up to 90%. Beyond two laboratories, the benefit is observed to drop sharply: Inclusion of a third or fourth laboratory further lowers the probability of consistent positive classifications, but the marginal gain in reliability is small relative to the associated increase in cost, effort, and organizational complexity. The model thus reveals an asymptotic relationship between the number of laboratories and classification certainty.

CONCLUSION: The analytical findings corroborate the empirical findings from VAH ring trials, demonstrating that the two-laboratory stipulation represents an optimal balance between statistical reliability and practical feasibility. Expanding beyond two laboratories provides minimal additional certainty and may increase disagreement rates due to compounding intralaboratory variation. Future standardization efforts should therefore prioritize the reduction of intralaboratory variability rather than expanding interlaboratory replication.