The general medical advice is that you should avoid drinking black coffee before a fasting blood test unless your healthcare provider specifically says it is okay. Black coffee contains caffeine and other bioactive compounds that can affect blood glucose, cholesterol, and lipid levels, potentially skewing the test results. Even though black coffee has no calories, its physiological effects like raising blood sugar, altering fat metabolism, and increasing heart rate can influence test accuracy. Plain water is encouraged during fasting periods instead. If coffee is consumed unintentionally before the test, it is important to inform the healthcare provider as it may affect interpretation or require rescheduling the test for accurate results.

However, some studies suggest that black coffee might not significantly impact certain fasting metabolic markers like triglycerides and glucose in healthy individuals, but this is not universally accepted as definitive guidance for all types of blood tests.

Therefore, for ensuring the most accurate fasting blood test results, it is safest to refrain from drinking black coffee before the test unless explicitly instructed otherwise by a healthcare professional.