Acid-Base

Blood Gas Analyzer

1Start with pH + PCO₂

Sample
pH
PCO₂
mmHg

2Add HCO₃⁻

HCO₃⁻
mEq/L

3Metabolic acidosis · classify the gap

Anion gap
Normal ≤13
mEq/L
Lactate
Optional · helps explain an elevated gap
mmol/L

NAGMA cause screen

Use the pattern to decide whether bicarbonate is being lost outside the kidney or renal acid excretion is impaired.

GI bicarbonate lossDiarrhea, ostomy, fistula
Chloride-rich fluidsRecent large-volume normal saline
Carbonic anhydrase inhibitorAcetazolamide or topiramate
Reduced kidney functionCKD can produce a normal-gap acidosis early
Urinary diversionUreterosigmoid or bowel urinary diversion

RTA finder

Use after NAGMA is established. Potassium and urinary NH₄ excretion are the fastest discriminators.

Serum K
High K strongly points toward type 4 RTA
mEq/L
Urine pH
Interpret during systemic acidosis
Urinary NH₄
Direct measurement preferred when available
Urine anion gap
Indirect NH₄ clue when direct NH₄ is unavailable
Fanconi pattern
Glucosuria without hyperglycemia, phosphaturia, uricosuria
Clinical context
Use only as a supporting clue
RTA comparison
Type 1 · Distal RTAFailure of distal H⁺ secretion
Serum KLow
Urine pH>5.5 during acidosis
Urinary NH₄Low
Urine anion gapOften positive
Typical clues
  • Nephrolithiasis or nephrocalcinosis
  • Hypocitraturia
  • Sjögren syndrome or other autoimmune disease
  • Amphotericin B or obstructive uropathy
Type 2 · Proximal RTAImpaired proximal bicarbonate reabsorption
Serum KLow
Urine pHOften <5.5 after serum HCO₃⁻ falls below the reabsorptive threshold
Urinary NH₄Usually preserved
Urine anion gapOften negative
Typical clues
  • Fanconi findings
  • Glucosuria without hyperglycemia
  • Phosphaturia or uricosuria
  • Tenofovir or monoclonal gammopathy
Type 4 RTAHypoaldosteronism or aldosterone resistance
Serum KHigh
Urine pHUsually <5.5
Urinary NH₄Low
Urine anion gapOften positive
Typical clues
  • Hyperkalemia
  • Diabetes or CKD
  • ACE inhibitor, ARB, MRA, or trimethoprim
  • Adrenal insufficiency
Compensation rules
Respiratory acidosisAcute HCO₃⁻ +1 per +10 PCO₂Chronic HCO₃⁻ +4 per +10 PCO₂
Respiratory alkalosisAcute HCO₃⁻ -2 per -10 PCO₂Chronic HCO₃⁻ -5 per -10 PCO₂
Metabolic acidosisWinter: expected PCO₂ = 1.5 × HCO₃⁻ + 8 ±2
Metabolic alkalosisExpected PCO₂ ≈ 40 + 0.7 × (HCO₃⁻ - 24) ±5

Metabolic Acidosis

High Anion Gap

AG = Na − Cl − HCO₃⁻

  • Lactic acidosis
  • Ketoacidosis
  • Renal failure
  • Toxic alcohols
  • Salicylates

Normal Anion Gap

Hyperchloremic metabolic acidosis

  • Diarrhea or other GI bicarbonate loss
  • RTA
  • Large-volume chloride-rich fluids
  • Acetazolamide or topiramate
  • CKD
Bicarbonate Deficit
Enter weight and bicarbonate values.

Metabolic Alkalosis

Urine Cl low

Usually chloride responsive

  • Vomiting or NG suction
  • Remote diuretic exposure
  • Post-hypercapnic alkalosis
  • Volume and chloride depletion

Urine Cl high

Think ongoing renal chloride loss or mineralocorticoid effect

  • Active loop or thiazide diuretic
  • Primary or secondary hyperaldosteronism
  • Bartter or Gitelman physiology
  • Severe hypokalemia or hypomagnesemia
Post-hypercapnic alkalosis: if PCO₂ falls rapidly, renal bicarbonate retention can persist after the respiratory acidosis has corrected.

Respiratory Disorders

Respiratory Acidosis

Primary rise in PCO₂ from alveolar hypoventilation.

  • COPD or severe asthma
  • OHS or OSA
  • Opioids or sedatives
  • Neuromuscular weakness
  • Chest wall or pleural restriction
  • Inadequate minute ventilation

Respiratory Alkalosis

Primary fall in PCO₂ from hyperventilation.

  • Hypoxemia or PE
  • Sepsis
  • Pain or anxiety
  • Pregnancy
  • Liver disease
  • Salicylates
  • Excess ventilator support