Acid-Base Balance for NEET PG 2026: Respiratory & Renal Physiology

Master ABG interpretation, Henderson-Hasselbalch, Winter's formula, anion gap, delta-delta and mixed disorders for NEET PG 2026 with workup tables and exam traps.

If pCO2 and HCO3 move in opposite directions (one up, one down), suspect a mixed disorder.

Step 3 — Verify compensation

If actual compensation differs from expected → mixed disorder.

Metabolic acidosis — two big buckets

Step 4 — Calculate anion gap

AG = Na − (Cl + HCO3); normal 8–12 mEq/L (some labs 6–14).

Always correct for albumin: corrected AG = measured AG + 2.5 × (4 − albumin g/dL). Hypoalbuminemia falsely lowers AG.

High AG metabolic acidosis — MUDPILES

• M — Methanol (formic acid; visual disturbance, fundoscopic edema)
• U — Uremia (renal failure)
• D — DKA (ketoacids — beta-hydroxybutyrate dominant)
• P — Propylene glycol, Paraldehyde
• I — INH (isoniazid), Iron, Ibuprofen
• L — Lactic acidosis (sepsis, shock, metformin, biguanides)
• E — Ethylene glycol (oxalate crystals in urine)
• S — Salicylates (mixed AG metabolic acidosis + respiratory alkalosis)

Non-AG (hyperchloremic) metabolic acidosis — HARDASS

• H — Hyperalimentation (TPN)
• A — Acetazolamide
• R — Renal tubular acidosis (RTA)
• D — Diarrhoea (GI HCO3 loss)
• A — Addison disease (mineralocorticoid deficit)
• S — Spironolactone, sulfa drugs
• S — Saline infusion (large volumes of NS)

Step 5 — Delta-delta and urinary anion gap

Delta-delta ratio

Delta-delta = (delta AG) ÷ (delta HCO3) = (measured AG − 12) ÷ (24 − measured HCO3)

Urinary anion gap (UAG)

UAG = (urine Na + urine K) − urine Cl

• Negative UAG → appropriate NH4+ excretion → GI loss (diarrhoea).
• Positive UAG → impaired NH4+ excretion → renal tubular acidosis.

RTA at a glance

Metabolic alkalosis

• Saline-responsive (urine Cl <20): vomiting, NG suction, diuretics (after stop), post-hypercapnia.
• Saline-resistant (urine Cl >20): primary hyperaldosteronism (Conn), Cushing, Bartter, Gitelman, Liddle, severe hypokalemia.
• Treat: stop diuretics, replace K and Cl, normal saline (saline-responsive); spironolactone (saline-resistant).

Respiratory acidosis and alkalosis

Respiratory acidosis (pCO2 >45)

• Acute: opioid overdose, neuromuscular weakness (myasthenia, GBS), severe asthma/COPD exacerbation, pneumothorax, ARDS.
• Chronic: COPD, obesity hypoventilation, kyphoscoliosis, central sleep apnoea.
• Treat: address ventilation (BiPAP, intubation), reverse cause.

Respiratory alkalosis (pCO2 <35)

• Causes: anxiety/hyperventilation, fever, sepsis (early), pneumonia, PE, salicylate toxicity, high altitude, pregnancy (progesterone), hepatic failure.
• Salicylate poisoning classically gives a mixed primary respiratory alkalosis + AG metabolic acidosis — a NEET PG favourite.

Mixed disorders — pattern recognition

Always suspect a mixed disorder if:

1. pH is normal but pCO2 and HCO3 are abnormal.
2. pCO2 and HCO3 move in opposite directions.
3. Compensation is greater or less than predicted.
4. Delta-delta is <1 or >2.

Classic clinical pictures

Worked NEET PG case

A 22-year-old with type 1 DM is brought in lethargic. ABG: pH 7.10, pCO2 18, HCO3 6, Na 140, Cl 100, K 5.5. Glucose 480 mg/dL.

1. pH 7.10 → acidemia.
2. HCO3 low + pCO2 low → metabolic acidosis with respiratory compensation.
3. Winter's: expected pCO2 = 1.5 × 6 + 8 = 17 ± 2 → measured 18 = appropriate compensation.
4. AG = 140 − (100 + 6) = 34 (high).
5. Delta-delta = (34 − 12) ÷ (24 − 6) = 22/18 = 1.2 → pure AG metabolic acidosis.

Diagnosis: Diabetic ketoacidosis with appropriate respiratory compensation, no superimposed metabolic alkalosis.

High-yield NEET PG MCQ traps

1. Compensation never normalises pH completely (except chronic respiratory alkalosis).
2. Salicylate = primary respiratory alkalosis + AG metabolic acidosis (mixed) — adults often present with normal pH.
3. Methanol → formic acid → optic disc oedema; treat with fomepizole + folic acid + dialysis.
4. Ethylene glycol → oxalate crystals (envelope-shaped) in urine; same antidote (fomepizole, dialysis).
5. DKA is dominantly beta-hydroxybutyrate (urine ketones may underestimate severity — they detect acetoacetate primarily).
6. Type 4 RTA is the only RTA with hyperkalemia.
7. Negative UAG = diarrhoea; positive UAG = RTA — high-yield split.
8. Bartter mimics loop diuretic; Gitelman mimics thiazide. Both have hypokalemic metabolic alkalosis with normal-to-low BP.
9. Permissive hypercapnia is acceptable in ARDS lung-protective ventilation (target tidal volume 6 mL/kg PBW).
10. Carbonic anhydrase inhibitors (acetazolamide) cause non-AG metabolic acidosis — used therapeutically in altitude sickness and glaucoma.

Recent updates

• Surviving Sepsis Campaign 2021 retains lactate-guided resuscitation; persistent lactate >2 mmol/L drives ongoing fluids/vasopressors.
• KDIGO 2024 AKI: acidosis treatment focuses on cause; bicarbonate replacement controversial except severe pH <7.1.
• DKA in India: rural type 2 patients on SGLT2 inhibitors are increasingly presenting with euglycemic DKA (glucose <200 mg/dL but high AG) — high-yield emerging vignette.
• Permissive hypercapnia in lung-protective ventilation is standard in ARDS guidelines.

Frequently Asked Questions

How do you interpret an ABG step by step?

First check pH (acidemia under 7.35, alkalemia over 7.45). Second identify the primary disorder by matching pCO2 and HCO3 direction with pH. Third apply the compensation formula (Winter's for metabolic acidosis, expected pCO2 for alkalosis). Fourth, calculate anion gap. Fifth, in raised AG metabolic acidosis check delta-delta for a co-existing disorder.

What is Winter's formula and when is it used?

Winter's formula predicts respiratory compensation in metabolic acidosis: expected pCO2 = (1.5 × HCO3) + 8 ± 2. If measured pCO2 is higher than expected, there is a co-existing respiratory acidosis; if lower, a co-existing respiratory alkalosis. It is one of the highest-yield single formulas in NEET PG physiology.

What is the anion gap and when is it elevated?

Anion gap = Na − (Cl + HCO3); normal 8–12 mEq/L. Elevated AG metabolic acidosis is caused by MUDPILES — methanol, uremia, DKA, propylene glycol, INH/iron, lactic acidosis, ethylene glycol, salicylates. Always correct AG for albumin: corrected AG = measured AG + 2.5 × (4 − albumin g/dL).

What is the delta-delta ratio?

Delta-delta = (delta AG) divided by (delta HCO3) — that is, (measured AG − 12) ÷ (24 − measured HCO3). Less than 1 means co-existing non-AG metabolic acidosis. Approximately 1 to 2 means pure AG metabolic acidosis. Greater than 2 means co-existing metabolic alkalosis. High-yield mixed-disorder finding.

What is the urinary anion gap useful for?

Urinary anion gap (UAG) = (urine Na + urine K) − urine Cl. Used in non-AG (hyperchloremic) metabolic acidosis. Negative UAG means high urinary NH4+ (appropriate renal response — GI loss like diarrhoea). Positive UAG means impaired NH4+ excretion (renal cause — RTA). NEET PG favourite to differentiate diarrhoea from RTA.

Written by: NEETPGAI Editorial Team Reviewed by: Pending SME Review Last reviewed: April 2026