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Osmolal Gap as a Biomarker in Kidney Injury: Focusing on the Differential Diagnosis of Metabolic Acidosis

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Biomarkers in Kidney Disease

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Abstract

Although numerous osmoles contribute to the measured value of osmolality in serum and urine, most clinical laboratories cannot measure these individually. The most part of osmolality of serum or urine is determined by five or six major effective osmoles. Formulae for calculating serum and urine osmolality derive from these osmoles. The difference between the measured and calculated osmolality in serum and urine is defined as the osmolal gap.

The serum osmolal gap is useful in the differential diagnosis of high anion gap metabolic acidosis and can be used as a screening test for intoxication with active osmoles, including ethanol or other toxic alcohols. In addition, serum osmolal gap can be increased in ketoacidosis, lactic acidosis, and advanced chronic kidney disease. The urine osmolal gap is correlated with ammonium (NH4 +) excretion and is useful in the differential diagnosis of normal anion gap metabolic acidosis. A decreased urine osmolal gap can be observed in patients with distal renal tubular acidosis, early chronic renal failure, and aldosterone deficiency or resistance.

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Abbreviations

Cl− :

Chloride

H+ :

Acid

HCO3 − :

Bicarbonate

K+ :

Potassium

Na+ :

Sodium

NH4 + :

Ammonium

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Author information

Authors and Affiliations

  1. Department of Internal Medicine, Hallym University Hangang Sacred Heart Hospital, 12, Beodeunaru-Ro 7-gil, Yeongdeungpo-Gu, Seoul, 150-719, Republic of Korea

    Jeonghwan Lee

  2. Department of Internal Medicine, Healthcare System Gangnam Center, Seoul National University Hospital, Gangnam Finance Center, 737 Yeoksam Dong, Gangnam Gu, Seoul, 135-984, Republic of Korea

    Nam Ju Heo

  3. Department of Internal Medicine, Seoul National University, College of Medicine, 101 Daehak-Ro Chongno-Gu, Seoul, 110-744, Republic of Korea

    Jin Suk Han

Authors
  1. Jeonghwan Lee
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  2. Nam Ju Heo
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  3. Jin Suk Han
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Corresponding author

Correspondence to Jin Suk Han .

Editor information

Editors and Affiliations

  1. Department of Biomedical Sciences, University of Westminster, London, United Kingdom

    Vinood B. Patel

Definitions

Osmolarity

The numbers of osmoles of solute per liter of solution (mmol or mOsmol/L).

Osmolality

Osmoles of solute per kg of solvent, expressed as mmol or mOsmol/kgH2O.

Measured osmolality

Osmolality which can be measured directly by osmometry.

Calculated osmolality

Osmolality which can be estimated by using formulae measuring the concentrations of the major effective solutes.

The osmolal gap

Difference between the measured and calculated osmolality.

Metabolic acidosis

Conditions with the gain of acid or loss of alkali due to the metabolic causes.

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Lee, J., Heo, N., Han, J. (2015). Osmolal Gap as a Biomarker in Kidney Injury: Focusing on the Differential Diagnosis of Metabolic Acidosis. In: Patel, V. (eds) Biomarkers in Kidney Disease. Springer, Dordrecht. https://doi.org/10.1007/978-94-007-7743-9_14-1

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  • DOI: https://doi.org/10.1007/978-94-007-7743-9_14-1

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  • Publisher Name: Springer, Dordrecht

  • Online ISBN: 978-94-007-7743-9

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