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Communications to the Editor |

Factors Influencing the Serum Osmolar Gap FREE TO VIEW

Vijo Poulose, MBBS, FCCP
Author and Funding Information

Affiliations: Changi General Hospital, Singapore,  University of Texas Medical Branch, Galveston, TX

Correspondence to: Vijo Poulose, MBBS, FCCP, Department of Medicine, Changi General Hospital, 2 Simei St 3, Singapore 529889; e-mail: vijopoulose@pol.net



Chest. 2002;122(1):381. doi:10.1378/chest.122.1.381
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To the Editor:

The case of isopropyl alcohol intoxication that was published in the “Pulmonary and Critical Care Pearls” section of the July 2001 issue of CHEST1 made for interesting reading.

However, there were a few comments that may need clarification. The authors stated that “an elevated serum osmolar gap (ie, > 10 mOsm/L) represents decreased serum water or the presence of low-molecular-weight solutes.” While the latter part of the statement is undisputed, the former cannot be made as a general statement. Hypernatremia caused by reduced water content can increase serum osmolarity but does not cause an increased osmolar gap, since the calculated and measured osmolarity are both high. The increased level of Na+ is reflected in the calculation for the osmolar gap, as follows: 2[Na] + [BUN]/2.8 + [glucose]/18.

The authors also mention that an increased osmolar gap “can be observed with ketoacidosis, hyperglycemia, mannitol infusion, and the ingestion of ethanol, methanol, ethylene glycol, or isopropyl alcohol.” To our knowledge, hyperglycemia does not cause an elevated osmolar gap for the same reasons mentioned in the previous paragraph.

A normal anion gap in the presence of urine ketones also could be seen in patients with starvation ketosis, since it is well-established that the ketone bodies produced in starvation states rarely cause an increase in the anion gap.23

Fahlen, M, Duarte, AG (2001) Gait disturbance, confusion, and coma in a 93-year-old blind woman.Chest120,295-297. [PubMed] [CrossRef]
 
Owen, OE, Morgan, AP, Kemp, HG, et al Brain metabolism during fasting.J Clin Invest1967;56,1589-1595
 
Cahill, GF, Herrera, MG, Morgan, AP, et al Hormone-fuel interrelationships during fasting.J Clin Invest1966;45,1751-1769. [PubMed]
 
To the Editor:

We thank Dr. Poulose for his remarks, and we appreciate his comments. Regarding his first point, we acknowledge that water loss increases serum osmolality but does not affect the serum osmolar gap, thus hypernatremia would produce an increased serum osmolality but would not result in a widened serum osmolar gap. As for the second point, we acknowledge that nonketotic hyperglycemia results in an increased serum osmolality, without a widened osmolar gap. With respect to the statement that starvation ketosis is associated with a normal anion gap, this statement is not entirely correct. There is a mild anion gap acidosis with starvation, but it is not as profound as that observed with diabetic ketoacidosis. During starvation ketosis, lipids become a significant source of fuel following ≥ 3 days of starvation. In addition, low insulin levels and increased serum glucagon concentrations result in the hepatic generation of ketones, and consequently incomplete oxidation of fatty acids and accumulation of ketones results in a mild metabolic acidosis.12 During a prolonged fast, the degree of ketosis is limited, likely due to a rise in insulin secretion that is secondary to ketonemia, thereby limiting the availability of free fatty acids.

References
Owen, OE, Caprio, S, Reichard, GA, et al Ketosis of starvation: a revisit and new perspectives.Clin Endocrinol Metab1983;12,359-379. [PubMed] [CrossRef]
 
Cahill, GF Starvation in man.N Engl J Med1970;282,668-675. [PubMed]
 

Figures

Tables

References

Fahlen, M, Duarte, AG (2001) Gait disturbance, confusion, and coma in a 93-year-old blind woman.Chest120,295-297. [PubMed] [CrossRef]
 
Owen, OE, Morgan, AP, Kemp, HG, et al Brain metabolism during fasting.J Clin Invest1967;56,1589-1595
 
Cahill, GF, Herrera, MG, Morgan, AP, et al Hormone-fuel interrelationships during fasting.J Clin Invest1966;45,1751-1769. [PubMed]
 
Owen, OE, Caprio, S, Reichard, GA, et al Ketosis of starvation: a revisit and new perspectives.Clin Endocrinol Metab1983;12,359-379. [PubMed] [CrossRef]
 
Cahill, GF Starvation in man.N Engl J Med1970;282,668-675. [PubMed]
 
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