Table 1. Differential features of renal... | American Society of Hematology

Table 1.

Differential features of renal proximal or distal tubular acidosis and the patient's values

	Normal	Patient	Distal RTA	Proximal RTA
During metabolic acidosis (spontaneous)
Plasma HCO₃⁻ (mEq/L)	18-22^*	15	Low	Low
Plasma K⁺(mEq/L)	3.5-4.5^*	3.5	Normal/low	Normal/low
Urine pH	< 5, 5^†	7.3	> 5.5	< 5.5
Urine anion gap (mEq/L)	−31 ± 23.5^†	+ 64	Positive	Negative
Ca²⁺/creat (u) (mmol/mmol)	< 0.7^*	1.2	High	Normal
Furosemide test
Minimum urine pH	4.99 ± 0.3^‡	6.6	> 5.5	< 5.5
During normal plasma HCO₃⁻ (after NaHCO₃load)
FE_HCO₃⁻(%)	< 3^*	4	< 5	> 10-15
U-B_PCO₂ (mm Hg)	> 20^†	15.8	< 20	> 20
Presence of nephrocalcinosis or lithiasis		Present	Common	Rare

	Normal	Patient	Distal RTA	Proximal RTA
During metabolic acidosis (spontaneous)
Plasma HCO₃⁻ (mEq/L)	18-22^*	15	Low	Low
Plasma K⁺(mEq/L)	3.5-4.5^*	3.5	Normal/low	Normal/low
Urine pH	< 5, 5^†	7.3	> 5.5	< 5.5
Urine anion gap (mEq/L)	−31 ± 23.5^†	+ 64	Positive	Negative
Ca²⁺/creat (u) (mmol/mmol)	< 0.7^*	1.2	High	Normal
Furosemide test
Minimum urine pH	4.99 ± 0.3^‡	6.6	> 5.5	< 5.5
During normal plasma HCO₃⁻ (after NaHCO₃load)
FE_HCO₃⁻(%)	< 3^*	4	< 5	> 10-15
U-B_PCO₂ (mm Hg)	> 20^†	15.8	< 20	> 20
Presence of nephrocalcinosis or lithiasis		Present	Common	Rare

Urine anion gap, (Na⁺ + K⁺ − Cl⁻); Ca²⁺/creat (u), urine calcium to creatinine ratio (mmol/mmol); FE_HCO₃⁻, fractional excretion of bicarbonate; U-B_PCO₂, urine to blood P_CO₂ gradient.

When a urine sample from a patient with hyperchloremic metabolic acidosis has a positive anion gap (Na⁺ + K⁺ − Cl⁻), a defect in distal urinary acidification is suspected. If the value of plasma K⁺ is normal or decreased, the demonstration of the inability to lower urine pH below 5.5, either after NH₄Cl loading or after furosemide administration, establishes the diagnosis of distal acidosis. The diagnosis is further supported by a low urine/blood P_CO₂ gradient (< 20 mm Hg) after NaHCO₃ loading. A fractional excretion of HCO₃⁻ at normal plasma HCO₃⁻concentration exceeding 5% of the filtered load indicates the presence of a proximal defect in HCO₃⁻ reabsortion. To complete the diagnostic work-up, we searched for a rise in Ca²⁺urinary excretion (urinary Ca²⁺/creatinine ratio > 0.7 mmol/mmol) and nephrocalcinosis by ultrasound. These features are commonly found in association with distal acidosis.

Data from Rodriguez-Soriano and Vallo.15

†

Data from Dalton and Haycock.14

‡

Data from Rodriguez-Soriano and Vallo.16

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