My creatinine is up and eGFR down after two bad weeks. How much of that is dehydration rather than kidney damage?

Question

Background: eight months on tirzepatide, uneventful until a bad fortnight of vomiting and diarrhoea after a dose increase, during which I ate very little and drank less than I should have. I had a panel drawn near the end of that fortnight because I felt terrible, and the numbers came back worse than my baseline. Marker Baseline, 14 months ago End of the bad fortnight Creatinine 72 µmol/L 118 µmol/L eGFR 92 mL/min/1.73m² 54 mL/min/1.73m² Urea 4.8 mmol/L 13.1 mmol/L Sodium 140 mmol/L 146 mmol/L Potassium 4.2 mmol/L 3.2 mmol/L Bicarbonate 25 mmol/L 30 mmol/L The eGFR drop looks alarming to me and I am now worried I have done permanent damage. But I also know I was genuinely dehydrated when this was drawn, and I have read that dehydration raises creatinine on its own. How do I read this panel? Is there anything in these specific numbers that distinguishes "volume depleted, will normalise" from "kidney injury"? And when should it be repeated?

Dr_Yusuf_Adeyemi · Accepted Answer

This panel has a coherent internal signature, and the signature is pre-renal, volume-responsive rather than intrinsic renal injury. That is not a diagnosis and it does not mean nothing happened; it means the pattern is the one that typically resolves with restored volume. Take it to a clinician for a repeat panel rather than to a forum for reassurance, but here is how the pattern reads and why. The five things in your numbers that point the same way 1. Urea rose proportionally far more than creatinine. This is the classic discriminator. Urea went from 4.8 to 13.1, a factor of 2.7. Creatinine went from 72 to 118, a factor of 1.64. Work the ratio in the units you have: Baseline: 4.8 / 72 = 0.067 mmol/µmol × 1000 = 66.7 Bad fortnight: 13.1 / 118 = 0.111 mmol/µmol × 1000 = 111.0 The ratio rose by two thirds. That is what pre-renal states do, and the reason is mechanistic rather than statistical: urea is reabsorbed in the proximal tubule and the collecting duct in proportion to sodium and water reabsorption, both of which are maximally upregulated when volume is low. Creatinine is filtered and essentially not reabsorbed. So volume depletion raises urea disproportionately. In intrinsic tubular injury the two rise roughly together, and the ratio stays flat or falls. In the conventional US units the same rule is expressed as a BUN-to-creatinine ratio above about 20:1 favouring pre-renal. 2. Sodium rose to 146. Free water loss in excess of sodium loss, which is what vomiting plus inadequate drinking produces. Intrinsic renal failure does not typically raise sodium; if anything the concentrating defect pushes the other way. 3. Bicarbonate rose to 30. Loss of gastric hydrogen chloride through vomiting produces a metabolic alkalosis, amplified by the contraction of extracellular volume itself. A rising bicarbonate is strong evidence for a gastric-loss mechanism. Note that intrinsic renal failure and ketoacidosis both push bicarbonate the other way, so if your bicarbonate had been 16 rather than 30 this would be a very different conversation. 4. Potassium fell to 3.2. Consistent with gastric losses plus secondary hyperaldosteronism from volume depletion plus renal potassium wasting driven by the alkalosis. Intrinsic renal failure with reduced filtration usually raises potassium. A low potassium alongside a high creatinine is a strongly pre-renal combination. 5. The clini

loss_on_drying · Answer

Quantifying the losses, because the volume involved in a fortnight of vomiting and diarrhoea is usually underestimated by a factor of two or three, and the electrolyte content differs by route in a way that matters.

Approximate composition of what you lose

Route	Typical volume	Sodium	Potassium	Chloride	Bicarbonate	Acid-base effect
Gastric (vomiting)	200-500 mL per episode	20-80 mmol/L	5-20 mmol/L	100-150 mmol/L	Low	Alkalosis, from loss of hydrogen and chloride
Small bowel / diarrhoea	Highly variable, can exceed 1 L/day	100-140 mmol/L	5-30 mmol/L	60-100 mmol/L	High, 30-70 mmol/L	Acidosis, from bicarbonate loss
Insensible (sweat, respiration)	0.5-1 L/day baseline, more with fever	Low	Low	Low	None	Free water loss, raises sodium

Two consequences of that table. First, someone vomiting has an alkalosis and someone with diarrhoea has an acidosis, and someone with both has whichever is winning. Your bicarbonate of 30 says gastric losses dominated, which is worth knowing because it also tells you potassium replacement matters more than bicarbonate does.

Second, the arithmetic on a bad fortnight. Suppose three vomiting episodes a week at 350 mL, plus one loose stool a day at 300 mL, plus a normal intake of about 2.2 L/day dropping to 1.0 L/day because drinking is unpleasant.

Extra output: 3 × 350 = 1,050 mL/week from vomiting, plus 7 × 300 = 2,100 mL/week from stool, so about 3.15 L/week.
Intake deficit: 1.2 L/day × 7 = 8.4 L/week.
Total weekly deficit: about 11.5 L, of which the intake shortfall is roughly three quarters.

The point of that arithmetic is not the precision, which is illusory. It is the ratio. Most of the deficit is not what came out; it is what did not go in. People fixate on the vomiting because it is dramatic, and the vomiting is the smaller term. This is why "drink more" is boring advice that happens to be the correct advice, and why the fluid diary is worth more than counting episodes.

On replacement composition: plain water replaces the volume and dilutes what remains, which is how someone with heavy losses and pure-water replacement ends up hyponatraemic instead of hypernatraemic. Oral rehydration solutions exist because they contain sodium at 45-75 mmol/L plus glucose, and the glucose is not there for calories; it is there because sodium-glucose co-transport in the small intestine is what drives absorption. That mechanism is why oral rehydration works at all and why sugar-free electrolyte tablets are less effective for absorption than the ones with a little glucose. Sports drinks are generally too dilute in sodium and too concentrated in sugar for this purpose, and the high osmolality can worsen diarrhoea.

None of this substitutes for assessment if you cannot keep fluids down. Oral replacement only works if oral works.

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answeredloss_on_drying47k13820 Jun 2026

The sodium-glucose co-transport point explains why the WHO formula has the composition it does and why homemade water plus salt underperforms. – Dr_Signe_Baldursdottir 6 months ago

2Three quarters of the deficit being reduced intake rather than output matches every case I have seen. – mz_4113 8 months ago

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stopper_core · Answer

One thing to add about how this presentation gets missed, since the question was drawn from an unusually well-documented case and most are not.

The people who end up with a significant renal event on this class overwhelmingly do not present saying they are dehydrated. They present with fatigue, dizziness, poor concentration and reduced exercise tolerance, all of which they attribute to the energy deficit, which is a completely reasonable attribution and is often partly correct. There is no moment that feels like an emergency. The specific reasons this is easy to miss:

Thirst is unreliable here. GLP-1 receptors are present in the subfornical organ, which is part of the circuitry governing thirst and fluid balance, and altered thirst is reported on this class. Relying on thirst to drive intake is unsafe in a population whose thirst signalling may be blunted.
Volume depletion and energy deficit share a symptom set. Fatigue, light-headedness, headache, poor performance. Both are common, they coexist, and there is no way to tell them apart by feel.
Weight loss is the goal, and volume loss looks like it. A 2 kg drop in a bad week reads as progress. It is water and glycogen and it will come back, but it removes the feedback that would otherwise signal something is wrong.
Reduced food intake means reduced fluid intake. Around 20-30% of daily water intake typically comes from food. Cutting intake in half cuts that too, so the fluid target rises exactly when eating becomes difficult. Almost nobody adjusts for this.
Creatinine can be falsely reassuring in this group for the muscle-mass and dietary-protein reasons in the accepted answer. Someone who has lost 25 kg may have a normal-looking creatinine over a genuinely reduced filtration rate.

The practical countermeasure is a floor rather than a symptom. Pick a daily fluid volume, measure it rather than estimating it, and treat falling below it as an event that needs addressing regardless of how you feel. Check urine colour and frequency as an objective read. And if you are going to have bloods drawn, draw them when you are well so that you have a real baseline, because the panel that is genuinely difficult to interpret is the one with nothing to compare it to.

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answeredstopper_core50k13826 Mar 2026

My creatinine is up and eGFR down after two bad weeks. How much of that is dehydration rather than kidney damage?

kidney-function

bloodwork

vomiting

hydration

3 Answers

The five things in your numbers that point the same way

On the eGFR figure specifically

When to repeat

The part that is not reassuring

Sigma-Aldrich - Certified Reference Materials

Approximate composition of what you lose

Your answer

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Marker	Baseline, 14 months ago	End of the bad fortnight
Creatinine	72 µmol/L	118 µmol/L
eGFR	92 mL/min/1.73m²	54 mL/min/1.73m²
Urea	4.8 mmol/L	13.1 mmol/L
Sodium	140 mmol/L	146 mmol/L
Potassium	4.2 mmol/L	3.2 mmol/L
Bicarbonate	25 mmol/L	30 mmol/L