Repeatability in measuring biomarkers for disease diagnosis is crucial for accuracy. An index of repeatability was developed based on variance components. The estimation of this index was derived using statistical methods and assessed for accuracy. Multiple observations on each subject are needed for valid estimation. The number of repeats required per subject depends on minimizing variance and cost constraints. The study found that the required sample size varies with cost and that repeatability is higher in females compared to males, as shown with Serum ALT data.