Serum osmol gap

Intoxication may present as inebriation and drowsiness similar to ethanol use. Other symptoms are vomiting, diarrhea, delirium and agitation, back and abdominal pain, and clammy skin. Toxic effects usually follow a latent period of several hours. Formate inhibits mitochondrial cytochromes resulting in neurotoxicity. Ocular signs include blurred vision, dilated pupils, and direct retinal toxicity with optic disc hyperemia and ultimately permanent blindness [91]. Cerebral hemorrhagic necrosis has been reported [92]. Severe poisoning may result in Kussmaul respiration, inspiratory apnea, coma, and death. Urine samples may have the characteristic smell of formaldehyde. An elevated serum osmolal gap from methanol will be evident early in presentation but may disappear after approximately 12 hours. At this time, an elevated anion gap metabolic acidosis from retained formate may be evident. 

Early laboratory findings include a high serum osmolal gap from the ethylene glycol. An extreme metabolic acidosis with greatly elevated anion gap follows, principally from glycolic acid [100]. Hypocalcemia and hyperkalemia may be evident and urinalysis may reveal calcium oxalate crystalluria, hematuria, and proteinuria. 

Geller RJ, Spyker DA, Herold DA, Bruns DE. Serum osmolal gap and ethanol concentration a simple and accurate formula. J Toxicol Clin Toxicol 1986 24 77-84. 

Glasser DS. Utility of the serum osmol gap in the diagnosis of methanol or ethylene glycol ingestion. Ann Emerg Med 1996 27 343-6. 

Garcia-Morales EJ, Cariappa R, Parvin CA. Osmole gap in neurosurgical-neurosurgical intensive care unit its normal value, calculation, and relationship with mannitol serum concentrations. Crit Care Med 2004 32(4) 986-991. 

Osmolar gap The difference between the measured serum osmolality and the calculated serum osmolality. 

This calculated value is normally 280-290 mOsm/L. Ethanol and other alcohols may contribute significantly to the measured serum osmolality but, since they are not included in the calculation, cause an osmolar gap ... 

An important clinical difference between the vapor pressure technique and the freezing point depression osmometer is the failure of the former to include in its measurement of total osmolality any volatile solutes present in the serum. Substances such as ethanol, methanol, and iso-propanol are volatile, and thus escape from the solution and increase the vapor pressure instead of lowering the vapor pressure of the solvent (water). This makes use of vapor pressure osmometers impractical for identifying osmolal gaps in acid-base disturbances (see Chapter 46). Thus use of this type of osmometer cannot be recommended for most clinical laboratories. 

The osmolality of stool "water wiU normally be that of serum (i.e., 290mosm/kg), but the contribution of electrolytes and of nonelectrolytes to the total osmolality will vary depending on the cause of the diarrhea. Fecal osmotic (osmolal) gap (FOG) expresses the difference between the theoretical normal osmolality (290 mosm/kg) and the contribution of Na and as follows ... 

An elevated serum osmolality and an increased osmolal gap can be of diagnostic vaiue in acute ethanol poisoning. 

This boy s calculated osmolality is approximately 206 mmol/kg. Thus, the osmolal gap is approximately 76 mmol/lkg. This has arisen because of his severe hyperlipidaemia which has caused pseudohyponatraemia. In severe hyperlipidaemia the plasma water concentration is decreased due to the abnormally high contribution of the lipoproteins to the plasma volume. As a result, the concentration of sodium per litre of serum is decreased as the sodium is not present in the lipoproteins but will have a normal concentration in the serum water. 

This simple formula only holds if the serum concentration of urea and glucose are within the reference ranges. If either or both are abnormally high, the concentration of either or both (in mmol/l) must be iiddcd in to give the calculated osmolality.. Sometimes there is an apparent difference betw een the measured and calculated osmolality. This is known as the osmolal gap (p. 17). 

If a low sodium concentration is suspicious, measure the serum osmolality. Know ing the glucose and urea concentration, the calculated osmolality can also be obtained. If the two results differ (that is, if there is a significant osmolal gap), the measured. sodium... 

B. Serum osmolality and osmolar gap. Serum osmolality may be measured in the laboratory with the freezing-point-depression osmometer or the heat-of-vaporization osmometer. Under normal circumstances the measured semm osmolality is approximately 290 mOsm/L and can be calculated from the results of the sodium, glucose, and BUN tests. The difference between the calculated osmolality and the osmolality measured in the laboratory is the osmo-lal gap, more commonly referred to as the osmolar gap (Table 1-22). 

Purssell RA et al Derivation and validation of a formula to calculate the contrt>utk>n of ethanol to the osmolal gap. Ann Emerg Med 2001 38(6) 653-659. [PMID 11719745] (Prospective evaluation of Intoxicated patients suggests that the serum ethanol concentration [in m dL] is approximately 3.7 times the osmolar gap.)... 

B. Other useful laboratory studies include electrolytes, BUN, creatinine, serum osmolality and osmolar gap (magnesium may elevate the osmolar gap), calcium, and ECG. 

B. Other useful laboratory studies include electrolytes, glucose, BUN, creatinine, calcium, ammonia, liver transaminases, bilirubin, prothrombin time (PT), amylase, serum osmolality and osmolar gap (see p 32 serum levels > 1500 mg/L may increase the osmolar gap by 10 mOsm/L or more), arterial blood gases or oximetry, and EGG monitoring. Valproic acid may cause a falsepositive urine ketone determination. 

Osmolar gap. Serum osmolaUty can be estunated from the concentrations of a few major blood constituents and can also be measured using coUigative properties (see Chapter 27). The osmolar gap (OG) is defined as the measured osmolality minus the calculated osmolality or... 

The marked increase in the serum urea with the modest increase in the serum creatinine would indicate the presence of pre-renal uraemia. Pyrexial patients are frequently hypercatabolic which will contribute to his high serum urea. His urine osmolality of 629 mmol/kg would support this, for if his pre-renal uraemia were purely due to dehydration his urine osmolality would be much higher. His low serum bicarbonate and high anion gap indicates that he has a metabolic acidosis. This acidosis will cause the potassium to move from the intracellular to the extracellular compartment. The reduction in his glomerular filtration rate results in his inability to maintain a normal serum potassium in the face of this efflux as both these factors contribute to his hyperkalaemia. 

B. Other useful laboratory studies include electrolytes, lactate, ethanol, glucose, BUN, creatinine, calcium, hepatic transaminases, urinalysis (for crystals and Wood s lamp examination), measured osmolality, arterial blood gases, and ECG monitoring. Serum beta-hydroxybutyrate levels may help distinguish ethylene glycol poisoning from alcoholic ketoacidosis, which may also cause increased anion and osmolar gaps. (Patients with alcoholic ke-... 

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