The present invention provides a method of determining the overall oxidative status of a body fluid or a tissue of a patient by measuring the oxidation-reduction potential (ORP) of the body fluid or tissue. The method has been found to be useful in the diagnosis, evaluation and monitoring of patients who have suffered a trauma (such as a head injury), patients suspected of being critically-ill, patients who have a viral infection, and patients suspected of having a myocardial infarction. The method has also been found useful in monitoring and evaluating exercise performance in patients. In addition, the method has been found useful in monitoring and evaluating stored blood products and patients who will receive such a product.