Renal, Fluids, Electrolytes & Acid-Base · Topic 20 of 22

Non-Anion Gap Metabolic Acidosis

Renal · acid–base

Non-Anion Gap Metabolic Acidosis

Bicarbonate lost, chloride kept: diarrhea, saline or a renal tubular acidosis. A negative urine anion gap points to the gut, a positive one to the kidney.

Key numbers

Urine anion gap: negative means GIUAG −30 to −50 → GI

01 · What it isLow bicarbonate, normal gap, high chloride

Normal anion gap (hyperchloremic) metabolic acidosis = low HCO₃⁻ with a normal anion gap and raised chloride, from gastrointestinal or renal bicarbonate loss or impaired renal acid excretion.

02 · CalculateUrine anion gap: negative means GI

Urine anion gap = urine Na⁺ + K⁺ − Cl⁻. A negative gap (about −30 to −50 mEq/L) means intact ammonium excretion — GI loss; a positive gap means renal loss, usually RTA.

03 · Don't confuseRTA: type 4 high K⁺, 1–2 low

Renal tubular acidosis: type 1 (distal) fails to excrete H⁺, urine pH > 5.5, low K⁺; type 2 (proximal) wastes HCO₃⁻, low K⁺; type 4 (low aldosterone effect) — the commonest — causes hyperkalemia.

Causes & risk factors

Why it happens

Diarrhea is the classic GI cause must knowGI bicarbonate loss is a leading cause of normal-gap acidosis: diarrhea, enteric fistulas, ileostomy or colostomy, biliary or pancreatic tube drainage, and ureterosigmoidostomy.

Large-volume saline causes hyperchloremic acidosis must knowRapid or large-volume 0.9% sodium chloride causes hyperchloremic normal-gap acidosis; Surviving Sepsis 2026 suggests balanced crystalloids over 0.9% saline for initial sepsis resuscitation (saline still suggested with traumatic brain injury).

Type 4 RTA: diabetes, ACE inhibitors, K⁺-sparing drugsSuspect type 4 RTA with unexplained persistent hyperkalemia and mild acidosis (HCO₃⁻ usually > 17): diabetic nephropathy, interstitial nephritis, ACE inhibitors or ARBs, spironolactone, amiloride, trimethoprim, heparin, NSAIDs, cyclosporine, adrenal insufficiency.

Drugs: acetazolamide, ion-exchange resins, chloride loadsDrug and infusion causes of normal-gap acidosis include acetazolamide (renal HCO₃⁻ loss), ion-exchange resins, ammonium chloride, arginine or lysine infusions, calcium chloride and magnesium sulfate.

Early kidney disease causes a normal-gap acidosisImpaired acid excretion in early kidney failure and tubulointerstitial disease causes a normal-gap acidosis; advanced kidney failure adds retained anions and a high gap.

Presentation

What you'll see

Type 1 RTA: urine pH > 5.5, stonesType 1 (distal) RTA: urine pH stays above 5.5 despite acidosis, HCO₃⁻ often < 15, hypokalemia, hypercalciuria and nephrolithiasis; causes include Sjögren syndrome, rheumatoid arthritis, amphotericin B, lithium and ifosfamide.

Type 2 RTA: Fanconi, myeloma, acetazolamideType 2 (proximal) RTA: HCO₃⁻ usually 12–20 with urine pH < 5.5 once HCO₃⁻ is depleted, often with Fanconi syndrome (glucose, phosphate, amino acids in urine); causes include myeloma light chains, acetazolamide and ifosfamide.

Diagnosis & monitoring

Work-up

Replace potassium before giving bicarbonatePotassium depletion is common (diarrhea, type 1 and 2 RTA) and alkali drives potassium lower; monitor potassium closely and give adequate potassium before IV bicarbonate, using potassium citrate when hypokalemia persists.

Do right now

In this order

First moves

Urine anion gap: negative means GIUAG −30 to −50 → GI

→

Replace potassium before giving bicarbonate

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Bicarbonate is appropriate in normal-gap acidosis

Why each step

Urine anion gap: negative means GIUrine anion gap = urine Na⁺ + K⁺ − Cl⁻. A negative gap (about −30 to −50 mEq/L) means intact ammonium excretion — GI loss; a positive gap means renal loss, usually RTA.

Bicarbonate is appropriate in normal-gap acidosisBecause the problem is lost bicarbonate, alkali therapy (sodium bicarbonate or citrate) is generally safe and appropriate in normal-gap acidosis, unlike high-gap acidosis; in RTA aim for HCO₃⁻ about 22–24 mEq/L.

Management

Orders

Type 4 RTA: lower potassium, sometimes fludrocortisoneTreat type 4 RTA hyperkalemia with volume expansion, dietary potassium restriction and a loop diuretic (eg, furosemide 20–40 mg); fludrocortisone helps some patients but can worsen hypertension, heart failure or edema.

Red flags & don'ts

Avoid harm

DKA recovery: normal-gap acidosis is not failure complicationHyperchloremic normal-gap acidosis often follows DKA treatment (lost keto-anions plus saline); judge resolution by ketones < 0.6 mmol/L and pH, not the gap, so subcutaneous insulin is not delayed.

High gap or normal gap?

Commonly confused

Mechanism	Added acid with an unmeasured anion
Serum chloride	Not raised
Main causes	Ketoacidosis, lactate, kidney failure, toxins
Next test	Ketones, lactate, osmolar gap, salicylate
Sodium bicarbonate	Controversial; usually only if pH < 7.0

More detail

Reference

Show 6 reference facts

Type 1 RTA: urine pH stays > 5.5 during acidosis or after an ammonium chloride load (normal kidneys reach < 5.2 within 6 hours). Type 2: fractional HCO₃⁻ excretion > 15% during bicarbonate infusion.

Type 2 RTA needs large alkali doses (10–15 mEq/kg/day of bicarbonate or citrate in divided doses), which increase urinary potassium loss; type 1 needs about 0.25–0.5 mEq/kg every 6 hours.

The urine anion gap detects ammonium only when it is excreted with chloride; with ketoacids, D-lactate or hippurate in the urine it misleads toward RTA, and the urine osmolal gap is the better estimate.

A high-gap acidosis comes from added acids with unmeasured anions (ketoacids, lactate, kidney failure, toxins) — evaluated with ketones, lactate, salicylate and the osmolar gap; see the Anion Gap Metabolic Acidosis topic.

In ureterosigmoidostomy the colon exchanges urinary chloride for bicarbonate and absorbs ammonium; toluene causes a high gap early that becomes a normal gap as metabolites are excreted.

Untreated chronic RTA causes nephrocalcinosis and stones (type 1), osteomalacia (type 2) and slowed growth in children, and type 1 often progresses to chronic kidney disease.

Pearl

Negative urine anion gap = GI loss: the kidneys are excreting ammonium chloride as they should.

Sources · Merck Manual Professional — Metabolic Acidosis (updated Dec 2025) · Merck Manual Professional — Renal Tubular Acidosis (Mar 2026) · Merck Manual Professional — Acid-Base Disorders · Surviving Sepsis Campaign: International Guidelines for Management of Sepsis and Septic Shock 2026 — SCCM recommendations page (fluid type; upgraded from 2021 rec 33) · Umpierrez GE et al. Hyperglycaemic crises in adults with diabetes: a consensus report (ADA/EASD/JBDS/AACE/DTS). Diabetologia 2024, doi 10.1007/s00125-024-06183-8 · Kamel KS, Halperin ML. Use of urine electrolytes and urine osmolality in the clinical diagnosis of fluid, electrolytes, and acid-base disorders. Kidney Int Rep 2021 · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).