AKT · Renal & urology/Fluid, electrolyte & acid-baselow yield

Acid-base disorders & renal tubular acidosis

Disturbance of pH from metabolic (HCO₃) or respiratory (CO₂) causes — classify by the anion gap

Overview

Disturbances of blood pH classified as metabolic or respiratory acidosis/alkalosis, read off an arterial/venous blood gas. The high-yield skills are the structured 4-step interpretation, splitting metabolic acidosis by the ANION GAP, and recognising the renal tubular acidoses. Med/Lab-science questions love a gas with a calculated anion gap and a compensation check.

Recognise

Read the gas in 4 steps: 1) pH (acidaemia/alkalaemia) 2) respiratory (CO₂) or metabolic (HCO₃) 3) compensation appropriate? 4) anion gap if metabolic acidosis
Metabolic acidosis — RAISED anion gap (MUDPILES: methanol, uraemia, DKA/ketones, propylene glycol, isoniazid, lactate, ethylene glycol, salicylates) vs NORMAL gap (GI/renal bicarbonate loss — diarrhoea, RTA)
RTA: type 1 (distal — can't acidify urine, hypokalaemia, stones/nephrocalcinosis), type 2 (proximal — bicarbonate wasting, Fanconi), type 4 (hypoaldosteronism — HYPERkalaemia)

Red flags

Severe acidaemia with haemodynamic compromise → treat the cause urgently (DKA, sepsis/lactate, toxic alcohols, AKI)
A high lactate / high osmolar gap → sepsis or toxic alcohol ingestion — escalate

Differentials & how to tell them apart

Raised-anion-gap metabolic acidosisMUDPILES — lactate (sepsis/ischaemia), ketones (DKA), urate (uraemia), toxins

Normal-anion-gap metabolic acidosisGI bicarbonate loss (diarrhoea) or renal tubular acidosis

Respiratory acidosis/alkalosisCO₂ retention (type 2 respiratory failure) or hyperventilation

Investigations

ABG/VBG (pH, CO₂, HCO₃, base excess, lactate), U&Es and chloride (calculate the ANION GAP = Na − (Cl + HCO₃)), glucose/ketones (DKA), osmolar gap (toxic alcohols), salicylate level; urine pH and electrolytes for suspected RTA.

Management

Interpret the gas (4 steps + anion gap) → treat the underlying cause

1Interpret the gas in 4 steps (pH → respiratory/metabolic → compensation → anion gap). For a metabolic acidosis, calculate the ANION GAP to split raised-gap (MUDPILES) from normal-gap (GI/renal bicarbonate loss).Gate: A normal-anion-gap metabolic acidosis with hypokalaemia + stones points to distal (type 1) RTA; with HYPERkalaemia points to type 4 (hypoaldosteronism) — the potassium splits them.
2Treat the underlying cause (DKA, sepsis/lactate, toxic alcohols, uraemia); oral bicarbonate for chronic metabolic acidosis/RTA; support ventilation for respiratory causes.

Treat the underlying cause — the priority — fluids/insulin for DKA, sepsis management, antidote for toxic alcohols, dialysis for severe uraemic acidosis

Oral sodium bicarbonate — for chronic metabolic acidosis (CKD, RTA); IV bicarbonate is reserved for specific severe situations

RTA: alkali + potassium management — type 1/2 — bicarbonate ± potassium citrate; type 4 — treat hyperkalaemia, fludrocortisone if hypoaldosteronism

Respiratory: support ventilation — treat the cause of CO₂ retention; controlled oxygen in COPD

Key points

4-step gas: pH → respiratory/metabolic → compensation → anion gap. Raised-gap metabolic acidosis = MUDPILES (lactate/ketones/uraemia/toxins); normal-gap = GI loss or RTA. RTA potassium splits the type: type 1/2 → hypokalaemia, type 4 → HYPERkalaemia. Always treat the cause.

Monitor & prognosis

Serial gas/lactate, electrolytes, the cause's response; urine pH in RTA.

Depends entirely on the cause; the gas is a window, not the disease.

Source: NICE CKS; cross-ref endocrine (DKA), acute_care