Investigations beyond standard operating procedure on internal standard response

Internal standard response is routinely monitored in regulated studies to accept or reject individual samples with outlier results due to potential sample processing or instrumentation errors, and processes are typically governed by standard operating procedures. However, acceptance or rejection of individual samples is not always sufficient. Internal standard response trends and substantial systemic differences between spiked and incurred samples using a method with an otherwise stable internal standard response should be investigated. Investigations may range from informal evaluations to detailed studies with formal investigation reports. Atypical internal standard response can be an indicator of systemic problems with a bioanalytical method and modification to allow for a relatively stable internal standard response across an analytical run may be essential.