Download Examination Osmolality Purpose of test The main reasons for

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
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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
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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
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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
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