Fasting before a blood test: what you can and can't do
రచన డా. విష్ణు అంజన్ నరెడ్డి, MD (జనరల్ మెడిసిన్)Professor of Medicine-Senior consultant physician (MD, General Medicine) · 4 నిమిషాల చదువుFasting before laboratory tests is often misunderstood. Some tests (like fasting sugar and lipid profile) require strict 8–12 hour fasting, while others (like CBC, HbA1c, and thyroid) do not require fasting at all. Drinking plain water is not only allowed but encouraged before every blood test.
Which tests need fasting and which do not?
- Fasting required (8–10 hours): Fasting Blood Sugar (FBS), Lipid Profile (Cholesterol & Triglycerides).
- No fasting needed: Complete Blood Count (CBC), HbA1c, Thyroid (TSH), Kidney Function (Creatinine), Liver Function, Urine Routine, Dengue NS1/IgM, Vitamin B12.
The golden water rule: please drink water!
Fasting means no food, tea, coffee, milk, or juices. However, drinking plenty of plain water is highly recommended. Being well-hydrated plumps up your veins, making blood collection fast, painless, and smooth.
What about morning prescription medicines?
Take your blood pressure and heart medications with a sip of water as normal. However, HOLD your morning diabetes tablets or insulin until immediately after the fasting blood sample is drawn, to prevent hypoglycemia.
Why fasting longer than 12 hours is harmful
Fasting for more than 14 hours stresses the liver to release stored glycogen, which can paradoxically falsely elevate your blood sugar readings. Stick to the 8 to 12 hour window.
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