Commercially prepared controls come in either assayed or unassayed forms. Assayed controls are tested by multiple methods before sale, and are sold with the results of the tests. Assayed controls:

• are more expensive than unassayed controls
• are used to evaluate accuracy and precision
• avoid laboratory errors in determining control values
• may only be suitable for specific methods or conditions

While the manufacturer's control values can be used to some extent to measure accuracy, the best measure of accuracy is certified reference material.

Unassayed controls are not tested by the manufacturer before they are sold. The control values for these materials must be determined by the individual laboratory. Unassayed controls:

• are less expensive than assayed controls
• are used to evaluate precision only
• avoid manufacturer error in determining control values
• control values are customized to the laboratory's own methods and conditions

A final note: although commercially available control materials are screened for hepatitis antigens and HIV antibodies, control materials should still be handled with precautions, since they contain biological materials and could contain infectious agents.