Serum cystatin C level alone provides GFR estimates that are nearly as accurate as serum creatinine level adjusted for age, sex, and race, thus providing an alternative GFR estimate that is not linked to muscle mass. An equation including serum cystatin C level in combination with serum creatinine level, age, sex, and race provides the most accurate estimates.

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Chronic aquatic toxicity test results are commonly analyzed with statistical hypothesis 'tests to generate summary statistics known as the no-observed-effect concentration (NOEC) and firstobserved-effect concentration (FOEC). These procedures address statistical differences among treatments but suffer several critical limitations. Use of concentration-response statistics to estimate minimal effect concentrations (i.e., EC values) from quanta1 and continuous data has advantages over a hypothesis-testing approach for generating a biologically relevant end point and an estimate of variability from toxicity tests. Estimation of the concentration-response statistic (EC, effective concentration) for continuous data is not straightforward but is possible with a variety of approaches. A statistical method for estimating EC values described here is based on a nonlinear regression estimation procedure. The usefulness of this method is demonstrated with continuous data from chronic toxicity tests with algae, fish, and invertebrate populations.