---
title: "Does dehydration affect blood test results?"
type: prep
url: "https://www.test-well.com/prep/hydration"
factor: Hydration
aliases:
  - drink water before blood test
  - dehydrated blood test creatinine
  - hydration lab results
reviewed_at: 2026-09-06
review_status: reviewed
date_published: 2026-09-05
---

# Does dehydration affect blood test results?

> 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.


## Which results does it affect?

| Marker | Direction | How long the effect lasts | What to do | Source |
| --- | --- | --- | --- | --- |
| Creatinine and BUN | raises | While dehydrated | Hydrate normally; water is allowed during a fast | [Testing.com (AACC) — Comprehensive metabolic panel](https://www.testing.com/tests/comprehensive-metabolic-panel-cmp/) |
| eGFR | lowers | While dehydrated (calculated from creatinine) | Repeat well-hydrated before acting on a low eGFR | [Testing.com (AACC) — Comprehensive metabolic panel](https://www.testing.com/tests/comprehensive-metabolic-panel-cmp/) |
| Hemoglobin, hematocrit | raises | While dehydrated | Hydrate normally | [Testing.com (AACC) — Comprehensive metabolic panel](https://www.testing.com/tests/comprehensive-metabolic-panel-cmp/) |
| Sodium | raises | While dehydrated | Hydrate normally | [Testing.com (AACC) — Comprehensive metabolic panel](https://www.testing.com/tests/comprehensive-metabolic-panel-cmp/) |
| Albumin, total protein | raises | While dehydrated | Hydrate normally | [Testing.com (AACC) — Comprehensive metabolic panel](https://www.testing.com/tests/comprehensive-metabolic-panel-cmp/) |
| Urine specific gravity / urinalysis | raises | While dehydrated | Normal fluid intake before a urine sample | [Testing.com (AACC) — Comprehensive metabolic panel](https://www.testing.com/tests/comprehensive-metabolic-panel-cmp/) |

## 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.

## Related

- [Do you need to fast before a blood test — and which tests does eating affect?](https://www.test-well.com/prep/fasting)
- [Does biotin affect blood test results?](https://www.test-well.com/prep/biotin)
- [How does alcohol affect blood test results?](https://www.test-well.com/prep/alcohol)
- [Should you exercise before a blood test?](https://www.test-well.com/prep/exercise)
- [All preparation guides](https://www.test-well.com/prep)
- [Lab visit guide](https://www.test-well.com/at-the-lab)

## Sources

- [Testing.com (AACC) — Comprehensive metabolic panel](https://www.testing.com/tests/comprehensive-metabolic-panel-cmp/)

---

Medically reviewed by Youmna DiStefano, MD (Medical Reviewer, TestWell); last reviewed 2026-09-06. Review policy: https://www.test-well.com/editorial-policy

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