Renal tubular acidosis (RTA) is a pathophysiological alteration of acid–base metabolism, characterised by the presence of hyperchloraemic metabolic acidosis, which is caused by renal loss of bicarbonate or a reduction in hydrogen ion excretion by the renal tubules.1 A suspicion of RTA is based on the clinical presentation of various signs and symptoms such as anorexia, vomiting, polyuria, polydipsia, delayed growth, muscle weakness, rickets, nephrocalcinosis and sensorineural deafness.2 The diagnosis is validated with laboratory examinations that should include the demonstration of hyperchloraemic metabolic acidosis, with a normal blood anion gap and a blood pH lower than 7.35, in patients with decompensated metabolic acidosis. In the case of secondary RTA, it is important to diagnose the systemic disease that is causing it.1 It should be noted that intestinal losses of bicarbonate, whether they are due to diarrhoea or a fistula, are a common cause of the same acid–base alterations, and so they should not be present when a diagnosis of RTA is made.

RTA has been reported to be overdiagnosed in Mexico,3,4 and it has been associated with allergy.4–6 For this reason, we conducted a study with the aim of documenting the diagnosis of RTA in children from different hospitals. A total of 170 children with a prior diagnosis of RTA were enrolled; the majority were receiving alkaline treatment. Treatment was suspended 5 to 7 days prior to the initial assessment, which consisted of a medical history, laboratory examinations and an assessment by the allergy department. A diagnosis of RTA was only confirmed in 3 patients (1.8%), one with distal RTA and 2 with RTA secondary to cystinosis, which were accompanied by Fanconi syndrome. None of them had an allergy. The rest of the patients’ prior diagnosis of RTA was erroneous; failure to thrive was caused by other conditions such as nutritional deficiency, Turner syndrome, giardiasis, coeliac disease, familial short stature, hypophosphataemic rickets, coenzyme Q10 deficiency or cardiomyopathy. The cases with RTA are described below.

Case 1: A 12-year-old eutrophic female patient with normal anthropometric measurements for her age and gender. She was diagnosed with RTA at one month of age, and since then she has been receiving a potassium citrate solution. On the third day of suspending alkaline treatment she had vomiting, metabolic acidosis, blood pH 7.22; [HCO3−] 10mmol/l; blood anion gap 11; K+ 2.2mEq/l; Cl− 114mEq/l; urine calcium/creatinine 1.38; urinary pH: 7.5. Alkaline treatment — bicarbonate and potassium — was restarted. Kidney ultrasound: grade III medullary nephrocalcinosis. Molecular study: mutation of the ATPV60A4 gene, not previously reported in the literature, which shall be the subject of a subsequent publication.

Case 2: A 22-month-old male patient with delayed growth since sixth months of age. At 11 months, he was diagnosed with distal ATR, with vomiting, polydipsia and polyuria, and he received treatment with a potassium citrate solution. Entry into the study: weight 7.88kg (p<3); height 71cm (p<3); weight/age 71%; weight/height 86.3%; height/age 91.8%; BMI Z-score: −1.47. Blood gases pH 7.39; [HCO3−] 16.2mmol/l; K+ 3.0mEq/l; HPO4− 2.4mg/dl; urine pH: 7.5; trace albumin, +glucosuria; microscopic haematuria, 45% tubular reabsorption of phosphate. Administration of furosemide: did not have urinary acidification. Ophthalmology: birefringent crystals in the cornea with a slit lamp. A diagnosis of infantile nephropathic cystinosis was considered. Molecular study of the CTNS gene (17p13, NG_012489.1 RefSeqGene): compound heterozygous genotype, predictor of severe form of infantile nephropathic cystinosis with deletion of 57kb, deletion of the first 10 exons, mutation more common in Caucasian, Mexican and Latin American patients7,8 with cystinosis and a minor deletion previously reported in European populations.9 Treatment with cysteamine bitartrate, phosphates, bicarbonate and potassium, with satisfactory evolution.

Case 3: A 20-month-old female patient with polyuria, polydipsia, anorexia and delayed growth. RTA was suspected, and she was referred to our institution without being treated. Weight 6.9kg (p<3); height 75.5cm (p<3); weight/age 60.9%; weight/height 73%; height/age 91.3%. Blood gases: pH 7.47; [HCO3−] 13.8mmol/l; K+ 3.6mEq/l; HPO4− 2.2mg/dl; Cl− 114mEq/l; urine pH 7.0; glucosuria 100mg/dl; +albumin. Ophthalmology: birefringent corneal cystine crystals. Molecular study of the CTNS gene: homozygous genotype, microdeletion of exon 12, which deleted amino acids 346–349 of the 7th transmembrane domain of cystinosin, which confirmed the diagnosis of infantile nephropathic cystinosis, reported in Europeans10. Treatment with cysteamine bitartrate, phosphates, bicarbonate and potassium.

We concluded that RTA is an uncommon tubulopathy, not associated with allergy, and confirmed that it is overdiagnosed in Mexico. We recommend a comprehensive paediatric approach in children with delayed growth, considering other diseases in addition to RTA, with special caution in the studies that are requested and their quality. When RTA is diagnosed, the presence of primary diseases with secondary RTA should be ruled out, and suitable guidance and nutritional support should be provided.