Numerous blood tests require fasting for up to 12 hours before the test, including lipid profile, renal function panel, glucose level, chem 7, basic metabolic profile, comprehensive metabolic profile and vitamin B-12, says Adams Memorial Hospital. Other fasting tests include tests for iron levels and liver function.
According to WellnessFX, consuming anything other than medications and water before a blood test may lead to inaccurate results. Strenuous exercise performed right before the test can also cause inaccuracies. For some blood tests, patients may need to abstain from alcohol and smoking for a certain time prior to testing, advises NHS Choices.