From acid to nephrocalcinosis to stones

Can you have an acidosis with normal serum bicarbonate? Of course youcan, it's just incomplete. Incomplete distal renal tubular acidosis (idRTA)that is.
RTA was first described in 1935, confirmed as a renal tubular disorderin 1946, and designated “renal tubular acidosis” in 1951 (see here for an excellentreview). Now it gets complicated, not only with regard to nomenclature but alsowith mechanisms.
I was a little surprised to hear that you can have distal RTA with anormal bicarbonate. It is just disguised. Patients with incomplete distal RTAhave persistently high urine pH but are still able to excrete acid under normalconditions (therefore the normal serum bicarbonate). However, in states of highacid loads (high protein diet, catabolic stress) they are unable to excretethat acid which then triggers alkali release from the bone and thus causesgreater bone resorption, therefore these patients have frequently osteopeniaand osteoporosis.
Distal RTA occurs with a number of conditions, amongst themclassically Sjogrens syndrome but also other autoimmune conditions. Cisplatinhas been mentioned as one of the causes of idRTA in this blog earlier. idRTA is a common cause of nephrocalcinosis - with or withoutstones - and it has a number of prominent victims as also mentioned in aprevious post.
idRTA can be diagnosed by induction of a systemic metabolic acidosisby means of acid loading. This is  commonly done with ammonium chloride (NH4Cl)but there is also a furosemide and fludrocortisone test that apparently causesless abdominal discomfort. Failure to acidify urine to a pH of less than5.3 is consistent with incomplete distal renal tubular acidosis. However,testing is a little bit tedious and therefore not commonly done. The urinarycitrate is commonly low in dRTA which contributes to nephrocalcinosis and stoneformation.

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