The evaluation of measurement uncertainty is not required by the European Union regulation or blood establishments’ laboratory tests. However, it is required for tests accredited by ISO 15189. Also, the forthcoming ISO 9001 edition requires “risk based thinking” with risk described as “the effect of uncertainty on an expected result”. ISO recommends GUM models for determination of measurement uncertainty, but their application is not intended for ordinal value measurements, such as what happens with screening test binary results. This article reviews, discusses and proposes concepts intended for measurement uncertainty of screening test results. The precision model focuses on cutoff level allowing the evaluation of the indeterminate interval using analytical sources of variance. The intervalis considered in the estimation of the seroconversion window period. The delta-value of patients and healthy subjects’ samples allows ranking two tests according to the probability of the two classes of indeterminate results: chance of false negative results and chance of false positive results (waste on budget).