Fasting Requirements for Laboratory Tests
No overnight fasting is required for any of the tests listed: hs-CRP, Thyroglobulin Panel, T3 Free, Thyroid Peroxidase Antibodies (TPO), Vitamin B12 and Folate Panel, T4 Free (FT4), TSH, or OmegaCheck.
High-Sensitivity C-Reactive Protein (hs-CRP)
- Fasting is not required for hs-CRP measurement. The CDC/American Heart Association guidelines explicitly state that hs-CRP can be measured "fasting or nonfasting" 1.
- Two independent measurements taken at least 2 weeks apart should be averaged for optimal risk stratification, but neither measurement requires fasting 1.
- Time of day for sample collection is not significant because diurnal variation for CRP is negligible 1.
Important caveat: The sample should be collected when the patient is metabolically stable, without obvious inflammatory or infectious conditions 1. If hs-CRP is ≥10 mg/L, search for an obvious source of infection or inflammation, discard that result, and repeat testing in 2 weeks 1.
Thyroid Function Tests (TSH, Free T4, Free T3, TPO Antibodies, Thyroglobulin Panel)
- Fasting is not required for any thyroid function tests. Multiple major Alzheimer's disease research cohorts including TARCC, ADCS, NIA Best Practices, and Alzheimer Center Amsterdam explicitly do not require fasting blood draws for these tests 1.
- TSH, free T4, free T3, TPO antibodies, and thyroglobulin measurements can all be performed on non-fasting samples 2, 3, 4.
Critical pitfall to avoid: Do not test thyroid function during acute metabolic stress, hyperglycemia, ketosis, or significant weight loss, as results may be misleading due to euthyroid sick syndrome 5. If initial tests are performed during metabolic instability, repeat them after achieving metabolic stability 5.
Additional consideration: While fasting itself is not required, short-term fasting (60 hours) can blunt the TSH response to TRH and decrease basal TSH values, potentially masking mild hypothyroidism in critically ill patients with poor caloric intake 6. However, this does not translate to a requirement for fasting before routine thyroid testing in stable outpatients.
Vitamin B12 and Folate Panel
- Fasting is not required for vitamin B12 or folate measurement. These tests are routinely performed on non-fasting samples in clinical practice.
- The relationship between folate levels and other markers (such as homocysteine in hypothyroid patients) has been studied using fasting samples, but this reflects research protocols rather than clinical necessity 7.
OmegaCheck®
- Fasting is not required for omega-3 fatty acid testing. Omega-3 index measurements reflect long-term incorporation into red blood cell membranes and are not significantly affected by recent food intake.
Practical Algorithm for Test Ordering
- Schedule blood draw at patient convenience - no fasting required for any of these tests
- Ensure metabolic stability - avoid testing during acute illness, infection, or metabolic derangement 1, 5
- For hs-CRP specifically: Plan for two measurements at least 2 weeks apart to average results 1
- For thyroid tests: If abnormal results are obtained during illness or metabolic stress, repeat after stabilization 5