From acid to nephrocalcinosis to stones

Can you have an acidosis with normal serum bicarbonate? Of course you can, it's just incomplete. Incomplete distal renal tubular acidosis (idRTA) that is.

RTA was first described in 1935, confirmed as a renal tubular disorder in 1946, and designated “renal tubular acidosis” in 1951 (see here for an excellent review). Now it gets complicated, not only with regard to nomenclature but also with mechanisms.

I was a little surprised to hear that you can have distal RTA with a normal bicarbonate. It is just disguised. Patients with incomplete distal RTA have persistently high urine pH but are still able to excrete acid under normal conditions (therefore the normal serum bicarbonate). However, in states of high acid loads (high protein diet, catabolic stress) they are unable to excrete that acid which then triggers alkali release from the bone and thus causes greater bone resorption, therefore these patients have frequently osteopenia and osteoporosis.

Distal RTA occurs with a number of conditions, amongst them classically Sjogrens syndrome but also other autoimmune conditions. Cisplatin has been mentioned as one of the causes of idRTA in this blog earlier. idRTA is a common cause of nephrocalcinosis - with or without stones - and it has a number of prominent victims as also mentioned in a previous post.

idRTA can be diagnosed by induction of a systemic metabolic acidosis by means of acid loading. This is  commonly done with ammonium chloride (NH4Cl) but there is also a furosemide and fludrocortisone test that apparently causes less abdominal discomfort. Failure to acidify urine to a pH of less than 5.3 is consistent with incomplete distal renal tubular acidosis. However, testing is a little bit tedious and therefore not commonly done. The urinary citrate is commonly low in dRTA which contributes to nephrocalcinosis and stone formation.

A recent study from Switzerland showed that 6.7% of 150 male recurrent calcium stone formers (RCSFs) had idRTA, i.e., 1 out of 15 male RCSFs can be expected to have idRTA. They therefore suggest that idRTA is overall underdiagnosed.

Posted by Florian Toegel