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Examination 17484 Purpose of test Osmolality The main reasons for measuring serum/plasma osmolality are i.screening for toxic ingestion ii.monitoring concentration of osmotically active agents iii.evaluation of hyponatremia Note: Ethylene glycol poisoning should be suspected irrespective of the blood ethylene glycol concentration or osmolal gap, in an intoxicated patient with anion gap acidosis, hypocalcaemia, urinary crystals, and nontoxic blood alcohol concentration Sample Blood Sample Tube/Container Adult- Yellow top or Green top Lithium Heparin Gel Paediatric- Green top Lithium Heparin Gel Sample Volume 4ml Minimum (see calculation of minimum volume) Special Precautions Time of collection should be given especially if paired with a urine osmolality sample Request Form: Clinical Chemistry & Haematology Requests Laboratory Biochemistry Biological reference range 275-295 mmol/Kg Clinical decision values Telephone limits <265 mmol/Kg >300 mmol/Kg osmolal gap >10mmol/Kg Factors affecting performance - Contamination from osmotically active substances - samples taken within 1 - 2 hours of ingestion may not fully reflect osmolal change, and maximum change in acid base develops over a period of up to 12 hours - osmolality may be normal in late presentation, therefore greater reliance should be place on clinical presentation Assessment of the osmolar gap as an indirect indication of poisoning (alcohols) is most useful when performed soon after ingestion. Osmolar gap has a low sensitivity and specificity which means the results must be interpreted with extreme caution. pre-existing negative osmolar gaps can lead to false negative results. Where there is a strong clinical suspicion of methanol or ethylene glycol ingestion antidote and quantification of levels strongly recommended Turnaround times: The Laboratory aims to report 90% of requests within the stated time from receipt Note: Printed documents are not controlled Page 1 of 4 Routine: 1 working day Water deprivation test: 1 hour Patient preparation No specific requirements Instructions for patient collected sample No specific requirements Sample transportation No specific requirements Special handling needs No specific requirements Patient consent required Implied consent Specific rejection criteria Generic rejection applies Additional information Minimum Retest Intervals- none available The most commonly used formula for the calculated osmolality is 2* Na +Urea+ Glucose. Please note however that in the investigation of poisoning TOXBASE recommend an alternative formula (see below) Hyponatraemia For guidance on the investigation of hyponatraemia see electrolyte disorders section on Intranet clinical guidelines (medical services) Ethylene Glycol/ Methanol Poisoning and the use of Osmolal Gap The osmolar gap is a method of assessing osmotically active constituents in serum (usually calcium, proteins and lipids). It correlates well with toxic alcohol concentrations (Hovda et al, 2004; Hunderi et al, 2006). It is based on the difference between the measured and calculated osmolality and should be calculated using the following formula: OSMOLAR GAP = [MEASURED OSMOLALITY CALCULATED OSMOLALITY] Formulae for calculated osmolality in poisoned patients (Hovda et al, 2004; Hunderi et al, 2006) 1) Ensure Sodium, Urea, Glucose , Ethanol and Osmolarity are all measured on the same sample and that all concentrations are in mmol/L 2) If ethanol is NOT suspected to have been ingested: Calculated osmolality = (glucose + urea + 1.86xNa) ÷ 0.93 3) If ethanol is suspected to have been ingested and an ethanol concentration (use mmol/L for this formula) is Note: Printed documents are not controlled Page 2 of 4 available: Calculated osmolality = (glucose + urea + ethanol + 1.86xNa) ÷ 0.93 4) Ethanol concentration in mmol/L = concentration in mg/dL ÷ 4.6 5) The normal range for osmolar gap according to the above formula is -9 to +19 mOsm/kg H2O (Aabakken et al, 1994) 6) Osmolality may be calculated using other formulae, but the above formula and reference range have the best evidence in poisoned patients 7) Ethylene Glycol has a molecular weight of 62 so if ethylene glycol poisoning is suspected multiply the remaining osmolal gap by 62 to give an approximate ethylene glycol level in mg/L 8) Methanol has a molecular weight of 32 so if methanol poisoning is suspected multiply the remaining gap by 32 to give an approximate methanol level in mg/L. Worked Example Sodium = 140mmol/L Urea = 5.0mmol/L Glucose = 5.0 mmol/L Ethanol = 100mg/dL Osmolarity – 400 mosm/kg Calculated osmolality = (glucose + urea + ethanol + 1.86xNa) ÷ 0.93 Ethanol conversion 100/4.6= 21.7 Calculated osmolality – (5.0 +5.0+21.7+260.4) /0.93 = 314 mmol/L Osmolal Gap = 400-314 = 86 Estimated concentration If ethylene glycol suspected = 86*62= 5332mg/L ethylene glycol Estimated concentration If methanol suspected = 86*32= 2752mg/L methanol Ethylene glycol has a molecular weight of 62 so an EG level of 500mg/L raises the osmolal gap by 8 mosmol/L Ethylene glycol has a half-life of 3-9 hours During Ethanol treatment T½ = 17-18 hours During Heamodialysis T½ = 2.5-3.5 hours Note: Printed documents are not controlled Page 3 of 4 Methanol has a molecular weight of 32 so a methanol level of 500mg/L raises the osmolal gap by 11 mosmol/L Methanol is cleared at a rate of 85mg/L/hr During ethanol treatment T½ = 43 hours During haemodialysis T½ = 2.5-3.5 hours References - Lab Tests Online - Pathology Harmony January 2011 Note: Printed documents are not controlled Page 4 of 4