Method validation is the process used to confirm that the analytical procedure employed for a specific test is suitable for its intended use. Results from method validation can be used to judge the quality, reliability and consistency of analytical results; it is an integral part of any good analytical practice.
Analytical methods need to be validated or revalidated

before their introduction into routine use;
whenever the conditions change for which the method has been validated (e.g., an instrument with different characteristics or samples with a different matrix); and
whenever the method is changed and the change is outside the original scope of the method.

SBTL have some specific guidelines for method validation we defines eight steps for validation:

Accuracy
Precision
Specificity
Limit of detection
Limit of quantitation
Linearity and range
Ruggedness
Robustness