Some blood tests need fasting, others do not. Getting this right matters because non-fasted readings of fasting-sensitive markers (glucose, insulin, triglycerides) can be misleading.
Tests that need fasting
Fasting glucose. Fasting insulin. Triglycerides (for a true fasted lipid panel). HbA1c does not technically need fasting but is often tested alongside markers that do.
Tests that do not need fasting
Thyroid hormones (TSH, free T4, free T3). Most sex hormones (testosterone is often tested fasting for consistency, but does not require it). Vitamin D, B12, folate. Ferritin and iron. Full blood count. CRP.
What "fasting" actually means
10 to 12 hours without food or drink other than water. Black coffee, tea without milk, and chewing gum technically affect some markers; safest to stick to water only. Continue any prescribed medication unless your clinician has said otherwise.
Why hard exercise also matters
Strenuous exercise within 24 to 48 hours of a blood test can affect liver enzymes, CK, inflammation markers and hormones. Most Chxhealth booking confirmations remind you of both fasting and exercise guidance for your specific panel. Chxhealth is a biomarker and genetic data provider. We do not diagnose, treat or prescribe. Our service supports your wellbeing journey alongside your healthcare professional.
Related biomarker guides
Frequently asked questions
Can I drink black coffee while fasting?
Strictly speaking, no. Coffee can affect some markers. Water only is the safest interpretation.
What if I forgot to fast?
Reschedule if the panel includes fasting-sensitive markers. A non-fasted glucose or triglycerides reading is not directly comparable to a fasting one.
About this article. Educational content. Chxhealth is a biomarker and genetic data provider. We do not diagnose, treat or prescribe. Our service supports your wellbeing journey alongside your healthcare professional. For medical advice about your health or results, please speak to a qualified clinician.