Assessing urine ammonium concentration by urine osmolal gap in chronic kidney disease

IgD lambda multiple myeloma causing hyperammonaemia with possibly reduced ammonium excretion

A man in his 70s presented with impaired consciousness due to hyperammonaemia while investigating multiple tumours in his brain, chest, spine and right adrenal gland. He did not have any disorders causing hyperammonaemia, such as liver dysfunction, urea-producing bacterial infection or any medications interfering with ammonium metabolism. Blood and urine tests in addition to tumour biopsy specimens confirmed immunoglobulin D lambda multiple myeloma. His general status responded to chemotherapy using bortezomib, dexamethasone and daratumumab, and he subsequently regained full consciousness and a normalised serum ammonia level. He, unfortunately, died of refractory multiple myeloma with hyperammonaemia. The autopsy specimen revealed lambda light-chain deposits in the distal tubule epithelium with cast precipitation and intact liver cells. Urine osmolality gap analysis suggested possibly reduced urinary ammonium excretion, but further investigation is necessary to elucidate the significance of pathological renal characteristics in multiple myeloma with hyperammonaemia.

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.