Evaluation of urinary acidification in children: Clinical utility

The kidney plays a fundamental role in acid-base homeostasis by reabsorbing the filtered bicarbonate and by generating new bicarbonate, to replace that consumed in the buffering of non-volatile acids, a process that leads to the acidification of urine and the excretion of ammonium (NH₄ ⁺). Therefore, urine pH (UpH) and urinary NH₄ ⁺ (UNH₄ ⁺) are valuable parameters to assess urinary acidification. The adaptation of automated plasma NH₄ ⁺ quantification methods to measure UNH₄ ⁺ has proven to be an accurate and feasible technique, with diverse potential indications in clinical practice. Recently, reference values for spot urine NH₄ ⁺/creatinine ratio in children have been published. UpH and UNH₄ ⁺, aside from their classical application in the study of metabolic acidosis, have shown to be useful in the identification of incomplete distal renal tubular acidosis (dRTA), an acidification disorder, without overt metabolic acidosis, extensively described in adults, and barely known in children, in whom it has been found to be associated to hypocitraturia, congenital kidney abnormalities and growth impairment. In addition, a low UNH₄ ⁺ in chronic kidney disease (CKD) is a risk factor for glomerular filtration decay and mortality in adults, even in the absence of overt metabolic acidosis. We here emphasize on the need of measuring UpH and UNH₄ ⁺ in pediatric population, establishing reference values, as well as exploring their application in metabolic acidosis, CKD and disorders associated with incomplete dRTA, including growth retardation of unknown cause.

Metabolic Acidosis or Respiratory Alkalosis? Evaluation of a Low Plasma Bicarbonate Using the Urine Anion Gap

Hypobicarbonatemia, or a reduced bicarbonate concentration in plasma, is a finding seen in 3 acid-base disorders: metabolic acidosis, chronic respiratory alkalosis and mixed metabolic acidosis and chronic respiratory alkalosis. Hypobicarbonatemia due to chronic respiratory alkalosis is often misdiagnosed as a metabolic acidosis and mistreated with the administration of alkali therapy. Proper diagnosis of the cause of hypobicarbonatemia requires integration of the laboratory values, arterial blood gas, and clinical history. The information derived from the urinary response to the prevailing acid-base disorder is useful to arrive at the correct diagnosis. We discuss the use of urine anion gap, as a surrogate marker of urine ammonium excretion, in the evaluation of a patient with low plasma bicarbonate concentration to differentiate between metabolic acidosis and chronic respiratory alkalosis. The interpretation and limitations of urine acid-base indexes at bedside (urine pH, urine bicarbonate, and urine anion gap) to evaluate urine acidification are discussed.

Non-Anion Gap Metabolic Acidosis: A Clinical Approach to Evaluation

Acid-base disturbances can result from kidney or nonkidney disorders. We present a case of high-volume ileostomy output causing large bicarbonate losses and resulting in a non-anion gap metabolic acidosis. Non-anion gap metabolic acidosis can present as a form of either acute or chronic metabolic acidosis. A complete clinical history and physical examination are critical initial steps to begin the evaluation process, followed by measuring serum electrolytes with a focus on potassium level, blood gas, urine pH, and either direct or indirect urine ammonium concentration. The present case was selected to highlight the differential diagnosis of a non-anion gap metabolic acidosis and illustrate a systematic approach to this problem.