Before your blood test
Does dehydration affect blood test results?
Short answer
Yes. Dehydration concentrates the blood, so hemoglobin, hematocrit, creatinine, BUN, sodium, albumin and total protein can all read high, and eGFR can read low — sometimes enough to suggest a kidney problem that isn't there. Drink water normally the day before and a glass or two the morning of the draw, even when fasting.
Medically reviewed by Youmna DiStefano, MD · Last reviewed September 2026
Which results does it affect?
| Marker | Direction | How long | What to do |
|---|---|---|---|
| Creatinine and BUN | raises | While dehydrated | Hydrate normally; water is allowed during a fast [source] |
| eGFR | lowers | While dehydrated (calculated from creatinine) | Repeat well-hydrated before acting on a low eGFR [source] |
| Hemoglobin, hematocrit | raises | While dehydrated | Hydrate normally [source] |
| Sodium | raises | While dehydrated | Hydrate normally [source] |
| Albumin, total protein | raises | While dehydrated | Hydrate normally [source] |
| Urine specific gravity / urinalysis | raises | While dehydrated | Normal fluid intake before a urine sample [source] |
Fasting doesn't mean no water
Water never breaks a fast for lab purposes. Skipping fluids along with food is the most common reason a healthy person's kidney panel comes back borderline, and it also makes the draw harder — a well-hydrated vein is easier to find. Over-drinking doesn't help: a liter of water in the waiting room won't improve results and can dilute a urine sample.
Common questions
How much water should I drink before a blood test?
Your normal intake the day before plus one or two glasses the morning of the draw. That's enough; there is no benefit to forcing fluids.
Can dehydration make my kidney function look bad?
Yes — creatinine rises and eGFR falls when you're dry. A single borderline eGFR is usually repeated well-hydrated before any conclusion.
Educational content, not a diagnosis. Lab results are one input; a clinician interprets them with your history and exam.