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plasma bicarbonate. In a healthy individual, this results in excretion of hydro-

gen ions to maintain normal blood pH. However, in patients with distal RTA,

excretion of hydrogen ions fails to occur, thereby leading to metabolic acidosis

and alkaline urine (urine pH >5.5). A decrease in plasma bicarbonate of

3–5 mEq/L along with urine pH >5.5 suggests the diagnosis of distal RTA. The

test should not be performed in the presence of liver disease, urinary tract infec-

tion, hypokalemia, or hypercalcemia.

76. How to confirm the diagnosis of RTA?

RTA should be suspected in a child in the presence of short stature, rickets–

osteomalacia, polyuria, or nephrocalcinosis–nephrolithiasis. The evaluation of

RTA includes renal function test, serum potassium, blood gas analysis, urine

pH, and urinary anion ga+p. The diagnosis of RTA is confirmed by the presence

of normal anion gap metabolic acidosis with normal renal function tests. If the

baseline urinary pH is <5.5 then the diagnosis of proximal RTA is confirmed

and if baseline urine pH is >5.5, bicarbonate loading test should be performed

to differentiate between proximal and distal RTA. In bicarbonate loading test,

oral sodium bicarbonate is administered at a dose of 2–4 mEq/Kg/day for

2–3 days, with the aim to normalize plasma pH and HCO 3 -. Fractional excre-

tion of HCO 3 − of >10–15 % suggests proximal RTA and <5 % distal RTA. The

differences in biochemical parameters between the two types of RTA are sum-

marized in the table given below.

Parameters

Proximal RTA Distal RTA

(Type 2) (Type 1)

Plasma anion gap Normal Normal

Serum potassium Low Low

Serum HCO 3 − mEq/L Usually 12–20 Usually <10

Urine pH

>5.5 (if HCO 3 - >20)

>5.5

<5.5 (if HCO 3 - <15)

Urine anion gap Positive Positive

Fractional excretion of HCO 3 − >10–15 % <5 %

Hypercalciuria Present Present

Hypocitraturia Absent Present

Aminoaciduria, glycosuria May be present Absent

77. How to treat rickets–osteomalacia associated with RTA?

The aim of therapy in a child with RTA is to correct metabolic acidosis in

order to promote mineralization and prevent further progression of skeletal

5 Rickets–Osteomalacia