July 23, 2017
Renal Tubular Acidosis Minimize

Renal tubular acidosis

Introduction

Acid excretion is one of the main roles of the kidney and congenital and acquired disorders of the nephron results in acidosis. Whilst chronic renal failure remains the commonest  from of acidosis, isolated tubular defects represent a more challenging diagnostic dilemma. The inherited forms are characterised by a hyperchloraemic acidosis with near normal GFR and plasma inorganic anions eg phosphate.

 

Classic distal RTA (Type I)

Incomplete distal RTA (Type I)

Voltage dependent distal RTA (Type I)

proximal RTA (Type II)

Type IV RTA (hypoaldosteronism)

Urine pH*

>5.5

>5.5

>5.5

<5.5

<5.5

Urine anion gap*

Positive

Negative

Positive

Negative

Positive

Fractional bicarbonate excretion**

< 5-10%

< 5-10%

< 5-10%

> 15%

5-15%

Frusemide test

Abnormal

Abnormal

Abnormal
(if reversible)

Normal

Normal

Urine calcium*

High

Normal / high

High

Normal

Normal

Urine citrate*

Low

Low / normal

Low

Normal

Normal

Renal stones

Common

Common

Common

Rare

Rare

Metabolic bone disease

Rare

Rare

Rare

Common

Rare

Other tubular defects

Rare

Rare

Rare

Common

Rare

Plasma potassium*

Normal / low

Normal

High

Normal / low

High

Plasma bicarbonate Normal/low can fall below 10 mmol/L

12-20 mmol/L

> 15 mmol/L

*      determined when plasma bicarbonate < 20 mmol/L
**    determined when plasma bicarbonate > 26 mmol/L

Urine pH

The ideal sample for measuring pH is a fresh early morning urine taken before breakfast. It is important to be certain that the urine is sterile as urea splitting organisms release ammonia and increase the pH.

Urine anion gap (UAG)

This is an indirect method for measuring urine [ammonia] and can be measured on a random urine sample. It is only valid when the urine pH < 6.5 as at greater pH, urine bicarbonate is a significant anion.

UAG = [urine Na] + [urine K] - [urine Cl]

The use of the UAG as an estimate of urine ammonium ion is disputed by some investigators (Kirschbaum et al)

Fractional bicarbonate excretion (FE (HCO3))

FE can be assessed on a random urine sample. Take care to ensure that the sample container is full and there is a minimum air space available for loss of bicarbonate by evaporation.

FE (HCO3) = (plasma HCO3 * urine creatinine) / (plasma creatinine * urine HCO3)

NB ensure that creatinine and bicarbonate are in the same units.

Hypokalaemia

Hypokalamia, or chronic acidosis may prevent normal urinary acidification and urine pH will be > 5.5, due to increased tubular ammoniagenesis. Hyponatraemia may also prevent acidification due to reduced cation available for exchange in the distal tubules.

Reference ranges for urinary analytes

Urine citrate (adults) 1.6 - 4.5 mmol / 24 hours or > 100 µmol / mmol creatinine on random urine.

Urine citrate (children) > 75 (males) > 177 (females) µmol / mmol creatinine on random urine.

Urine ammonia (adults) 36 - 99 µmol / min / 1.73 m2

Urine ammonia (children under 15 years) 49 - 119 µmol / min / 1.73 m2

References

  • Kirschbaum B, Sica D, Anderson P. Urine electrolytes and the urine anion and osmolar gaps. J Lab Clin Med 1999;133:597-604.
  • Penney MD, Oleesky DA. Renal tubular acidosis. Ann Clin Biochem 1999;36:408-22.
  • Emmett M. Pathophysiology of renal tubular acidosis and the effect on potassium balance. UpToDate Aug 26, 2013

JHB 17 April 2014

  

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