How RTA is tested on Step 1

A vignette gives a non-anion gap acidosis, a potassium value, a urine pH, and a drug or systemic disease. Your job is to name the type and the mechanism.

AcidosisLow HCO₃⁻,low pHAnion gapNormal (8–12)Urineanion gapNegative → GIlossPositiveRenal: RTAPotassiumHigh → type 4
Workup. Diarrhea is the most common cause of a non-anion gap acidosis; RTAs are the renal causes.

The three types compared

Location of the defect explains everything else.

FeatureType 1 (distal)Type 2 (proximal)Type 4 (hypoaldosteronism)
DefectH⁺ secretion by α-intercalated cellsHCO₃⁻ reabsorption in proximal tubuleAldosterone deficiency or resistance
Urine pH> 5.5< 5.5 (once HCO₃⁻ falls below threshold)< 5.5
Serum K⁺LowLowHigh
ComplicationsCalcium phosphate stones, nephrocalcinosisRickets, osteomalaciaHyperkalemia
TreatmentAlkali (bicarbonate or citrate)High-dose alkali plus potassium; treat causeLower K⁺; fludrocortisone if aldosterone-deficient

Type 3 (mixed proximal and distal) is rare and usually omitted.

Nephron defectProximal (type 2)HCO₃⁻ wastedDistal (type 1)H⁺ not secretedAldosterone (type 4)↑ K⁺ → ↓ NH₃
Where the defect is. In type 4, hyperkalemia reduces ammonia synthesis, limiting acid excretion.

Fanconi syndrome and stones

Proximal tubule dysfunction affects more than bicarbonate.

Fanconi (proximal)Glucosuria with normal serum glucoseAminoaciduriaPhosphaturia → ricketsBicarbonaturiaUricosuriaDistal RTAAlkaline urineHypercalciuriaLow urine citrateCalcium phosphate stonesNephrocalcinosisvs
Bones vs stones. Proximal RTA causes bone disease through phosphate wasting; distal RTA causes stones in alkaline urine.

Urine anion gap

The urine anion gap estimates urinary ammonium, which carries chloride.

SituationUrinary NH₄⁺Urine anion gap
DiarrheaHigh (kidneys respond normally)Negative (Cl⁻ exceeds Na⁺ + K⁺)
Distal (type 1) RTALowPositive
Type 4 RTALowPositive

Urine anion gap = urine Na⁺ + urine K⁺ − urine Cl⁻. It is less reliable in proximal RTA and in volume depletion.

Worked example

Work through the gaps, then the urine pH and potassium.

functions Worked example — classify the RTA

A woman with Sjögren syndrome has a kidney stone. Serum: Na 140, Cl 116, HCO₃⁻ 15, K 3.0. Urine pH 6.6; urine anion gap positive. Which RTA is this?

  1. Anion gap = 140 − (116 + 15) = 9: normal anion gap acidosis.
  2. Positive urine anion gap: the kidney is not excreting ammonium, so the cause is renal.
  3. Urine pH > 5.5 despite acidosis, with hypokalemia, indicates failure of distal H⁺ secretion.
  4. Sjögren syndrome and calcium phosphate stones fit type 1 (distal) RTA.

Answer. Type 1 (distal) RTA. Treat with alkali therapy such as potassium citrate.