DKA anion gap explained: from high gap to hyperchloraemic acidosis
In diabetic ketoacidosis the anion gap starts high. During treatment it can close while the patient is still acidotic, because ketoanions leave in the urine and chloride takes their place.
Chapters (11)
Explain the high anion gap acidosis of DKA from ketoanions, and why a normal anion gap (hyperchloraemic) phase can appear during treatment.
The short version
- DKA is a high anion gap metabolic acidosis from the ketoanions acetoacetate and beta-hydroxybutyrate.[1][2]
- Ketoanions filtered by the kidney leave in the urine with sodium or potassium: potential bicarbonate that metabolism can no longer give back.[2][3]
- Large volumes of normal saline keep the urine flowing and supply chloride, which takes the place of the lost ketoanions.[2][3]
- During treatment the picture can change from a high gap acidosis to a hyperchloraemic, normal gap acidosis.[2][3]
- A gap that closes faster than bicarbonate rises means anions were lost, not converted.[2]
The DKA anion gap puzzle: a normal gap, but still acidotic
The DKA anion gap starts high and can close while the patient is still acidotic. A 22-year-old with type 1 diabetes arrives in diabetic ketoacidosis. Sodium is 131, chloride 95 and bicarbonate 7. The anion gap is 29: a big gap, as expected.
By morning the sugar is down and the ketones are clearing, yet bicarbonate is still low and the gap is normal. Where did the ketoanions go, and why is he still acidotic?
This lesson explains the DKA anion gap: the high gap acidosis of ketoacids, and why a normal gap, hyperchloraemic phase can appear during treatment.[2][3]
The tool: the anion gap
Normal about 12 mEq/L (K⁺ is not included).[5]
When an acid adds a new anion, bicarbonate falls and the gap rises, roughly one for one. The rise in the gap should approximately equal the fall in bicarbonate.[2]
The mechanism: ketoacids and the high anion gap in DKA
In DKA, insulin is severely lacking, and glucagon, catecholamines and cortisol rise. Fat goes to the liver, which makes acetoacetic acid and beta-hydroxybutyric acid.[1][2]
These are strong organic acids. They give up their hydrogen ions, and bicarbonate buffers them. So bicarbonate falls, the unmeasured anions rise, and the result is a high anion gap metabolic acidosis.[1][2]
Check with the opening patient: 131 − (95 + 7) = 29, a high gap.
The kidney: ketoanions lost as potential base
Ketoanions are filtered by the kidney, and they leave in the urine paired with sodium or potassium.[2][3]
This is the key idea. Every ketoanion lost in the urine is potential bicarbonate that metabolism can no longer give back: it is potential base, thrown away.[2][3]
Compare what happens when ketones are burned. Normally each one gives back a bicarbonate: new bicarbonate is made as the ketones are metabolised.[2]
Saline and chloride: what takes the ketoanions' place
Meanwhile we give saline. Big volumes of normal saline keep the urine flowing, so more ketoanions are lost. The saline also carries chloride, and chloride takes the place of the lost ketoanions.[2][3]
A normal anion gap metabolic acidosis is also called hyperchloraemic acidosis.[4]
Next morning: the gap fell more than bicarbonate rose
Now the morning labs from the opening patient: sodium 137, chloride 108, bicarbonate 17.
- Arrival: 131 − (95 + 7) = 29
- Morning: 137 − (108 + 17) = 12, normal
- The gap fell by 29 − 12 = 17
- Bicarbonate rose by 17 − 7 = 10
- The missing 7 is the non-gap deficit
The missing 7 is the non-gap deficit: ketoanions lost in the urine, plus chloride from the saline. A gap that closes faster than bicarbonate rises means anions were lost, not converted.[2][3]
Two phases: high gap acidosis, then hyperchloraemic acidosis
So the picture changes over hours: a high gap acidosis first, then a hyperchloraemic, normal gap acidosis.[2][3]
It is common. One review reports that 20% or more of DKA patients have a non-gap component.[3] Sources differ on how much is saline chloride and how much is ketoanion loss. Both matter.[2][3]
Treatment, in brief: fluids, insulin once potassium is safe, and potassium replacement.[1]
| Phase | Anion gap | Why |
|---|---|---|
| On arrival | High | Ketoanions (acetoacetate, beta-hydroxybutyrate) are unmeasured anions |
| During treatment | Normal; chloride high | Ketoanions lost in urine, saline chloride in their place |
Practice question
Original · USMLE styleA 24-year-old man with type 1 diabetes mellitus is admitted with diabetic ketoacidosis. On arrival, serum glucose is 612 mg/dL, Na⁺ 136 mEq/L, Cl⁻ 100 mEq/L and HCO₃⁻ 9 mEq/L. He receives a large volume of 0.9% saline and an insulin infusion. Fourteen hours later he feels better. Temperature is 37.0 C (98.6 F), pulse is 84/min, respirations are 16/min and blood pressure is 124/76 mm Hg. He is alert and not vomiting. Laboratory studies now show:
| Serum glucose | 168 mg/dL |
|---|---|
| Serum Na⁺ | 139 mEq/L |
| Serum K⁺ | 4.2 mEq/L |
| Serum Cl⁻ | 113 mEq/L |
| Serum HCO₃⁻ | 15 mEq/L |
| Arterial pH (room air) | 7.31 |
| Arterial PCO₂ (room air) | 31 mm Hg |
Common questions
Why is the anion gap high in DKA?
Insulin lack drives the liver to make acetoacetic acid and beta-hydroxybutyric acid. Bicarbonate buffers their hydrogen ions, so bicarbonate falls and the unmeasured ketoanions raise the gap.
Why can the anion gap become normal during DKA treatment?
Ketoanions are lost in the urine as potential bicarbonate, and chloride, partly from saline, takes their place. The result can be a hyperchloraemic, normal gap acidosis.
What does it mean when the gap closes faster than bicarbonate rises?
It means anions were lost, not converted. In the worked example the gap fell by 17 but bicarbonate rose by only 10; the missing 7 is the non-gap deficit.
How common is a non-gap component in DKA?
One review reports that 20% or more of DKA patients have a non-gap component. Sources differ on how much is saline chloride and how much is ketoanion loss.
Transcript
Read the full narration, by chapter662 words
0:00High gap on arrival, normal gap by morning
A 22-year-old with type one diabetes arrives in diabetic ketoacidosis. Sodium 131, chloride 95, bicarbonate seven. The anion gap is 29. A big gap, as expected. By morning the sugar is down and ketones are clearing, yet bicarbonate is still low, and the gap is normal. Where did the ketoanions go, and why is he still acidotic?
0:27Today's objective
The objective today: explain the high anion gap acidosis of DKA, and why a normal anion gap, high-chloride phase can appear during treatment.
0:38The tool: the anion gap
First, our tool. The anion gap is sodium, minus chloride and bicarbonate together. We use a normal of about 12. When an acid adds a new anion, bicarbonate falls and the gap rises, roughly one for one.
0:54The mechanism: ketoacids and the high gap
Now the mechanism. In DKA insulin is severely lacking, and glucagon, catecholamines and cortisol rise. Fat goes to the liver, which makes acetoacetic acid and beta-hydroxybutyric acid. These are strong organic acids. They give up their hydrogen ions, and bicarbonate buffers them. So bicarbonate falls, the unmeasured anions rise, and the result is a high anion gap metabolic acidosis.
1:25The kidney: ketoanions lost as potential base
Now watch what the kidney does. Ketoanions are filtered, and they leave in the urine paired with sodium or potassium. This is the key idea. Every ketoanion lost in the urine is potential bicarbonate that metabolism can no longer give back. It is potential base, thrown away. Normally, when ketones are burned, each one gives back a bicarbonate. New bicarbonate is made as the ketones are metabolised.
1:54Saline and chloride
Meanwhile we give saline. Big volumes of normal saline keep the urine flowing, so more ketoanions are lost. The saline also carries chloride, and chloride takes the place of the lost ketoanions.
2:08Next morning: the gap fell more than bicarbonate rose
Now that morning: Sodium 137, chloride 108, bicarbonate 17. The gap is 137 minus the sum of 125, which is 12. Normal. The gap fell by 17, but bicarbonate rose by only 10. The missing seven is the non-gap deficit: ketoanions lost in the urine, plus chloride from the saline.
2:34Two phases, how common, and treatment in brief
So the picture changes over hours: high gap acidosis first, then a high-chloride, normal gap acidosis. It is common. One review reports that 20% or more of DKA patients have a non-gap component. Sources differ on how much is saline chloride and how much is ketoanion loss. Both matter. Treatment, in brief. Fluids, insulin once potassium is safe, and potassium replacement.
3:05Practice question
Your turn. A 24-year-old man with type one diabetes is admitted with diabetic ketoacidosis. On arrival his glucose is 612, sodium 136, chloride 100, bicarbonate nine. He is treated with a large volume of normal saline and an insulin infusion. Fourteen hours later he feels better. Glucose 168, sodium 139, potassium 4.2, chloride 113, bicarbonate 15. Arterial pH 7.31, PaCO2 31. Which best explains his current acid-base status? A, persistent ketoacidosis from inadequate insulin. B, lactic acidosis from tissue hypoperfusion. C, acute respiratory acidosis from hypoventilation. D, a normal anion gap metabolic acidosis after ketoanion loss and saline. E, a high anion gap metabolic acidosis with respiratory alkalosis. Pause here and decide.
4:20The answer, option by option
The gap is 139 minus the sum of 128, which is 11. Normal. A is out: persistent ketoacidosis would keep the gap high, and his is 11. B is out: lactate is also an unmeasured anion, so it would raise the gap. C is out: acute respiratory acidosis needs a high PaCO2, and his is 31, low. E is out: Winters' formula gives 30 and a half, give or take two. His 31 sits inside the window, and a high gap is absent. So the answer is D. Gap 27 on arrival, 11 now, bicarbonate up only six. Ketoanions lost in urine, chloride in their place.
5:10High-yield recap and next lesson
Three things to keep. One: DKA is a high anion gap acidosis from the ketoanions, acetoacetate and beta-hydroxybutyrate. Two: ketoanions lost in the urine, plus saline chloride, can leave a high-chloride, normal gap acidosis during treatment. Three: a gap that closes faster than bicarbonate rises means anions were lost, not converted. Next up: potassium and pH, transcellular shifts.
Sources
Numbers in the text, e.g. [1], point here- Diabetic Ketoacidosis (DKA). MSD Manual Professional Version. msdmanuals.com/professional/endocrine-and-metabolic-disorders/diabetes-mellitus-and-disorders-of-carbohydrate-metabolism/diabetic-ketoacidosis-dka
- Brandis K. Acid-base Physiology, 8.02 Ketoacidosis (LibreTexts). med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Book%3A_Acid-base_Physiology_(Brandis)/08%3A_Major_Types_of_Metabolic_Acidosis/8.02%3A_Ketoacidosis
- Kraut JA, Kurtz I. Treatment of acute non-anion gap metabolic acidosis. Clin Kidney J 2015. pmc.ncbi.nlm.nih.gov/articles/PMC4377741
- Metabolic Acidosis. MSD Manual Professional Version. msdmanuals.com/professional/endocrine-and-metabolic-disorders/acid-base-regulation-and-disorders/metabolic-acidosis
- Acid-Base Disorders (anion gap, Winters' formula). MSD Manual Professional Version. msdmanuals.com/professional/endocrine-and-metabolic-disorders/acid-base-regulation-and-disorders/acid-base-disorders
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): liver, kidney. 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-018.
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