Why is fasting generally necessary before a chemical analysis could be done on a patient?

Cross reaction: Certain foods and beverages can contain products that after break down can mimick certain classes of drugs. This can give you a false positive result and cause problems in interpreting the lab results.

Target fixed in time: The normal ( and abnormal) values of any test are established by making the collection standards the same. Your blood sugar or fat content will differ just after a meal with a milkshake than it would after an overnight fast, since the body takes time to process the meal. Some tests are not as affected by this factors, some are, so whether or not fasting is needed depends on the testing