Metabolic alkalosis: generation, maintenance and urine chloride
A metabolic alkalosis needs both an event that raises bicarbonate and a kidney that keeps it. Urine chloride shows which kind of maintenance is at work and whether saline will help.
Chapters (9)
Explain why a metabolic alkalosis needs both a generating event and a renal maintenance factor, and use urine chloride to separate chloride-responsive from chloride-resistant causes.
The short version
- Generation adds the bicarbonate; maintenance, a kidney that keeps it, makes the alkalosis last.
- The generating event and the maintaining factor are often different things.
- Urine chloride below about 20 mEq/L means chloride-responsive: volume-depleted, and saline fixes it.
- Urine chloride above about 20 mEq/L means chloride-resistant, usually from aldosterone excess or potassium loss, and saline typically does not fix it.
- Read the number with the drug history: the cut-off varies by source, current diuretics can raise it, and it is not diagnostic in advanced kidney disease.
The puzzle: why does the kidney keep the bicarbonate?
A man's blood gas shows a pH of 7.52 and a bicarbonate of 38 mEq/L. Normal bicarbonate is about 24 mEq/L. That is a metabolic alkalosis: a primary rise in bicarbonate, so the pH goes up.[1][2]
Here is the puzzle. Healthy kidneys can dump extra bicarbonate into the urine, so why is his still high? The answer is that every metabolic alkalosis needs a generator and a maintainer, and urine chloride tells us which kind of maintainer we have.
Step 1: how metabolic alkalosis is generated
Generation answers the question: how did the bicarbonate go up? Either the body gained alkali, for example from a bicarbonate infusion, or it lost acid.[1][2]
- Alkali gain: for example, a bicarbonate infusion.
- Acid loss from the stomach: vomiting or suction.
- Acid loss through the kidney: some diuretics.
Why generation alone is not enough
A healthy kidney filters a lot of bicarbonate and can excrete the excess, so a pure generating event would fade away. If the alkalosis persists, something is making the kidney hold on to bicarbonate. That is the maintenance.[1][2]
Maintenance factors in metabolic alkalosis
Four factors do this: volume depletion, chloride depletion, potassium depletion and a low GFR. Excess aldosterone does it too.[2]
| Maintenance factor | Effect on the kidney |
|---|---|
| Volume depletion | The proximal tubule reclaims more bicarbonate, along with sodium.[2] |
| Chloride depletion | With little chloride available, the kidney exchanges less chloride for bicarbonate, so bicarbonate stays in the blood.[1][2] |
| Potassium depletion | Pushes bicarbonate reabsorption up.[1][2] |
| Excess aldosterone | Pulls sodium in and pushes potassium and hydrogen ions out into the urine, so bicarbonate is kept.[1][2] |
| Low GFR | Named as a maintenance factor.[2] |
Urine chloride in metabolic alkalosis: responsive vs resistant
To find the maintainer, measure the urine chloride. The cut-off used in this lesson is about 20 mEq/L.[1][2]
| Chloride-responsive | Chloride-resistant | |
|---|---|---|
| Urine chloride | Below about 20 mEq/L | Above about 20 mEq/L |
| Volume status | Short of volume and chloride; the kidney clings to chloride, so almost none reaches the urine | Often volume-expanded; a chloride shortage is not the maintainer |
| Causes | Vomiting; diuretics taken in the past | Excess aldosterone or other mineralocorticoids; a severe potassium deficit; Bartter syndrome; a diuretic being taken right now |
| Saline | Replaces volume and chloride, and the kidney lets the extra bicarbonate go | Does little, because chloride was never the problem |
| Treatment | Replace volume and chloride | Treat the hormone or the potassium |
Three cautions when reading urine chloride
- Sources differ slightly: some use below 10 as the sure sign of volume depletion, so values in between are a grey zone.[1][2]
- A patient on a diuretic today can have a high urine chloride, so read the number with the drug history.[2]
- In advanced kidney disease, stage 4 or 5, these urine values are not diagnostic.[1]
Worked case: a metabolic alkalosis with urine chloride 85
A 52-year-old man has had 4 months of headaches, muscle cramps and weakness. He denies vomiting and takes no medicines. Blood pressure is 164/102 mm Hg. Na⁺ 143, K⁺ 2.8, Cl⁻ 98, HCO₃⁻ 35 (mEq/L); arterial pH 7.49; urine Cl⁻ 85 mEq/L.
- Name the disorderHigh pH and high bicarbonate: a metabolic alkalosis.
- Use urine chloride85 is well above 20, so the alkalosis is chloride-resistant.[1][2]
- Rule out the responsive causesVomiting, past diuretics and suction all cause a chloride-responsive picture with a low urine chloride.[1][2] He has no vomiting, no medicines and no tube.
- Use the blood pressureBartter syndrome is chloride-resistant but starts in childhood and gives a normal or low blood pressure.[3] His pressure is high.
- Name the maintainerExcess aldosterone holds on to bicarbonate, wastes potassium and raises the blood pressure.
Practice question
Original · USMLE styleA 52-year-old man comes to the physician because of headaches, muscle cramps and generalized weakness for 4 months. He has had no vomiting or diarrhea and takes no medications. Pulse is 78/min and blood pressure is 164/102 mm Hg. There is no orthostatic change in blood pressure. Examination shows no edema and no other abnormalities. Laboratory studies show:
| Serum Na⁺ | 143 mEq/L |
|---|---|
| Serum K⁺ | 2.8 mEq/L |
| Serum Cl⁻ | 98 mEq/L |
| Serum HCO₃⁻ | 35 mEq/L |
| Serum urea nitrogen | 14 mg/dL |
| Serum creatinine | 0.9 mg/dL |
| Arterial pH (room air) | 7.49 |
| Arterial PCO₂ (room air) | 47 mm Hg |
| Urine Cl⁻ | 85 mEq/L |
Common questions
What is the difference between generation and maintenance in metabolic alkalosis?
Generation is how the bicarbonate went up: alkali gain, or acid loss from the stomach or kidney. Maintenance is the kidney holding on to bicarbonate so the alkalosis persists.
What does urine chloride tell you in metabolic alkalosis?
It separates the two kinds of maintainer. Below about 20 mEq/L is chloride-responsive (volume-depleted); above about 20 is chloride-resistant, often from aldosterone excess or potassium loss.
Which causes of metabolic alkalosis are chloride-responsive?
The classic causes are vomiting and diuretics taken in the past. Saline replaces volume and chloride, and the kidney lets the extra bicarbonate go.
Which causes of metabolic alkalosis are chloride-resistant?
Excess aldosterone or other mineralocorticoids, a severe potassium deficit, Bartter syndrome, or a diuretic being taken right now. Saline does little; the hormone or the potassium is treated.
When is urine chloride unreliable?
Some sources treat values between 10 and 20 as a grey zone, a current diuretic can raise the value, and in stage 4 or 5 kidney disease the urine values are not diagnostic.
Transcript
Read the full narration, by chapter688 words
0:00The puzzle: why does the kidney keep the bicarbonate?
A man's blood gas shows a pH of 7.52, and a bicarbonate of 38. Normal bicarbonate is about 24. Here's the puzzle: healthy kidneys can dump extra bicarbonate into the urine. So why is his still high? Today: every metabolic alkalosis needs a generator and a maintainer, and urine chloride tells us which kind of maintainer we have.
0:25Step 1: generation (alkali gain, H+ loss from stomach or kidney)
Metabolic alkalosis means a primary rise in bicarbonate, so the pH goes up. Step one is the generation: how did the bicarbonate go up? Either the body gained alkali, for example from a bicarbonate infusion, or it lost acid. Acid is lost from the stomach, with vomiting or suction, or through the kidney, with some diuretics.
0:49Why generation alone is not enough
But generation alone is not enough. A healthy kidney filters a lot of bicarbonate and can excrete the excess, so a pure generating event would fade away. If the alkalosis persists, something is making the kidney hold on to bicarbonate. That is the maintenance.
1:08Maintenance factors: volume, chloride, potassium, GFR, aldosterone
Four factors do this: volume depletion, chloride depletion, potassium depletion, and a low GFR. Excess aldosterone does it too. Volume depletion makes the proximal tubule reclaim more bicarbonate, along with sodium. With little chloride available, the kidney exchanges less chloride for bicarbonate, so bicarbonate stays in the blood. Low potassium also pushes bicarbonate reabsorption up. And aldosterone pulls sodium in, and pushes potassium and hydrogen ions out into the urine, so bicarbonate is kept. Notice the trap: the event that generated the alkalosis and the factor that sustains it are often different things.
1:52Urine chloride: responsive vs resistant
So how do we find the maintainer? We measure urine chloride. Group one: urine chloride below about 20 milliequivalents per litre. We call it chloride-responsive. The body is short of volume and chloride, so the kidney clings to chloride and almost none reaches the urine. The classic causes are vomiting, and diuretics taken in the past. Give saline, replace the volume and the chloride, and the kidney lets the extra bicarbonate go. Group two: urine chloride above about 20. This one is chloride-resistant. The patient is often volume-expanded, and the maintainer is not a shortage of chloride. It is an excess of aldosterone or other mineralocorticoids, a severe potassium deficit, Bartter syndrome, or a diuretic being taken right now. Saline does little here, because chloride was never the problem. We treat the hormone or the potassium.
2:52Three cautions: grey zone, current diuretics, advanced CKD
Three cautions. Sources differ slightly: some use below 10 as the sure sign of volume depletion, so values in between are a grey zone. A patient on a diuretic today can have a high urine chloride even while volume-depleted, so read the number with the drug history. And in advanced kidney disease, stage four or five, these urine values are not diagnostic.
3:17Practice question
Your turn. A 52-year-old man has four months of headaches, muscle cramps and weakness. He denies vomiting, and takes no medicines. His blood pressure is 164 over 102. Serum sodium 143, potassium 2.8, chloride 98, bicarbonate 35. Arterial pH 7.49. Urine chloride 85. What's the most likely cause? A, surreptitious vomiting. B, diuretic use that stopped last month. C, primary hyperaldosteronism. D, nasogastric suction. E, Bartter syndrome. Pause here and decide. This is a metabolic alkalosis: high pH, high bicarbonate. The urine chloride is 85, well above 20: chloride-resistant. A is out: vomiting causes a chloride-responsive picture, with a low urine chloride. B is out: past diuretics are chloride-responsive too, and he takes no medicines. D is out: suction also loses chloride and fits the low urine chloride group, and he has no tube. E is out: Bartter syndrome is chloride-resistant, but it starts in childhood and gives a normal or low blood pressure, not 164 over 102. So the answer is C: excess aldosterone holds on to bicarbonate, wastes potassium, and raises the blood pressure.
4:56High-yield recap
Three things to keep. One: generation adds the bicarbonate, but maintenance, a kidney that keeps it, makes it last. Two: urine chloride below about 20 means chloride-responsive, the volume-depleted type that saline fixes. Three: above about 20 means chloride-resistant, usually from aldosterone excess or potassium loss, and saline will not fix it.
5:23Next lesson: vomiting alkalosis
Next up: vomiting, and why it gives a low-chloride, low-potassium alkalosis.
Sources
Numbers in the text, e.g. [1], point here- MSD Manual Professional Version. Metabolic Alkalosis. msdmanuals.com/professional/endocrine-and-metabolic-disorders/acid-base-regulation-and-disorders/metabolic-alkalosis
- LITFL (Life in the Fast Lane). Metabolic Alkalosis. litfl.com/metabolic-alkalosis/
- LITFL (Life in the Fast Lane). Bartter Syndrome. litfl.com/bartter-syndrome/
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations: Servier Medical Art (smart.servier.com), licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/); adapted (recoloured, outlined, resized): nephron, urine sample container, stomach. Intravenous drip icon: Health Icons (healthicons.org), CC0. All other diagrams are original; example lab values are illustrative.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-REN-010.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01ABG interpretation, made simple5:35
- EP 02Anion gap explained: high vs normal gap metabolic acidosis5:25
- EP 03Winters' formula: the expected PaCO₂ in metabolic acidosis5:10
- EP 04Delta ratio: find the hidden second acid–base disorder5:09
- EP 05New bicarbonate: how the kidney excretes acid with ammonium5:23
- EP 06High anion gap metabolic acidosis causes, sorted by the anion5:54
- EP 07Osmolar gap: how toxic alcohols hide from the anion gap5:31
- EP 08Urine anion gap: telling diarrhea from RTA5:33
- EP 09Renal tubular acidosis: tell RTA types 1, 2 and 4 apart5:24
- EP 10Metabolic alkalosis: generation, maintenance and urine chloride5:30
- EP 11Vomiting metabolic alkalosis: why the urine turns acid5:36
- EP 12Bartter vs Gitelman: NKCC2, NCC and urine calcium5:21
- EP 13Respiratory acidosis compensation: acute vs chronic5:22
- EP 14Respiratory alkalosis compensation: acute vs chronic5:33
- EP 15Metabolic alkalosis compensation: the expected PaCO₂5:12
- EP 16Salicylate poisoning acid-base: a classic mixed disorder5:31
- EP 17Type A vs type B lactic acidosis: split it by mechanism5:33
- EP 18DKA anion gap explained: from high gap to hyperchloraemic acidosis5:42
- EP 19Acidosis and hyperkalemia: how pH shifts potassium5:43
- EP 20ABG interpretation in 6 steps: the acid-base algorithm5:32
- EP 21Acid-base practice cases: three ABG vignettes, one algorithm5:44