You should not drink coffee while fasting for bloodwork. Even black coffee, without any additives, can potentially skew blood test results because caffeine and components in coffee may affect certain measurements, especially those related to sugar metabolism. Additionally, coffee is a diuretic, which can cause dehydration and make it harder to draw blood or affect blood concentration. Only plain water is recommended during fasting before blood tests to ensure accurate results unless your healthcare provider specifically instructs otherwise.
