What is a control material and why are low and high controls used?

• A. A material with known concentration used to verify assay performance; high and low controls test range.
• B. A patient specimen used for calibration.
• C. A test that yields yes/no only.
• D. A standard used for housekeeping.

Answer: (A)

Control materials are samples with known concentrations of an analyte that are run alongside patient specimens to verify that the assay is performing correctly. Using a low and a high control checks the assay across its measuring range: the low control tests accuracy at the lower end, and the high control tests accuracy at the upper end. This helps detect shifts in instrument accuracy, reagent quality, or procedural issues, ensuring results stay within established QC limits. Controls are treated like patient samples and are not used to set the calibration curve—that role belongs to calibrators. If a control falls outside the acceptable range, it signals a problem that needs investigation before patient results are reported.