Endocrinology

Why is Sodium (Na+) times two in Osmolar gap?

The serum osmolality is calculated by the concentrations of the different effective osmoles which compose it. The formula:

Calculated Sosm   =   (2  x  serum [Na])  +  [glucose]  + [urea] (SI)

Calculated Sosm   =   (2  x  serum [Na])  +  [glucose, in mg/dL]/18  + [blood urea nitrogen, in mg/dL] (US)

WHY SODIUM TIMES TWO? The serum sodium is multiplied by two in order to account for the negatively charged solutes that must accompany it in the serum to maintain net neutrality in the body (i.e chloride and bicarbonate).

REFERENCES

  1. Rose BD, Post TW. Clinical Physiology of Acid-Base and Electrolyte Disorders, 5th ed, McGraw-Hill, New York 2001.
Get Medical Pearls directly to your inbox every week!
Weekly posts with high yield medical knowledge, directly to your mailbox!
Dr. C Humphreys

Internal Medicine

Recent Posts

Older Age and Obstructive Sleep Apnea (OSA): Mechanism

Older age is recognized as a risk factor for obstructive sleep apnea.[1] But why? Suspected…

1 month ago

How does Weight Affect Obstructive Sleep Apnea (OSA): Mechanism

Being overweight (BMI ≥ 25 kg/m2)/Obese (BMI ≥ 30 kg/m2) is one of the most…

1 month ago

Nocturia and OSA: Mechanism

Nocturia is a common symptom associated with obstructive sleep apnea. [1] Img Cred: Am J…

1 year ago

ASD vs. VSD Murmur Difference

ASD (Atrial Septal Defect) Wide, Fixed split S2 (in contrast to the normal variation in…

1 year ago

Mechanism of a Mixed Apnea

Mixed apneas are characterized by absent respiratory effort and airflow in the first section of…

2 years ago

How Does Hypothyroidism Cause Hypoventilation?

Although rare, the differential diagnosis of hypoventilation and hypercapnia respiratory failure includes hypothyroidism. It is…

2 years ago

This website uses cookies.