An observation or value that represents the background level of a measurable quantity.
A baseline in medicine is information found at the beginning of a study or other initial known value which is used for comparison with later data. The concept of a baseline is essential to the daily practice of medicine in order to establish a relative rather than absolute meaning to data.
Example: if a patient with kidney failure (whose creatinine is usually 3.0 mg/dL) suddenly has a creatinine of 5.0 mg/dL, then his creatinine is out of his normal. For that person with kidney failure, absolute normal no longer applies because he will never again be able to obtain an absolutely normal creatinine level (0.5-1.2 mg/dL) with kidneys that no longer function properly.
It is the initial observation or value of a measurable quantity at the time of diagnosis or assessment (before an intervention) against which later tests will be compared.
The baseline and the response values refer to the same individual or system.
Baseline assessment: it is performed during the design phase of a surveillance plan of action. It provides information on the existing situation, forms the basis for the development of the plan of action, and provides baseline data against which prospective changes in the surveillance system are progressively assessed or measured.
Baseline data: it is collected at the outset of implementation of a surveillance system or of strengthening activities, or a set of indicators that have been identified to monitor and evaluate the performance of a surveillance and response system. For example, the baseline mortality rate (or non-crisis mortality rate) is mortality rate before the crisis.