Osmolality (calc) = 2 x Na + Glucose + urea (all measurements in
mmol/L) seems to work clinically
or
http://www.google.com/search?q=osmolar+gap&ie=UTF-8&oe=UTF-8
I think the clinical need for is a quick method to "suggest"
accumulated unusual solute, so that it is possible to rapidly identify
which patients need further investigation and which don't.
http://www.sydpath.stvincents.com.au/tests/Osmolality.htm says:
"The normal osmolar gap is up to 10 mmol/L and values in excess of this
usually indicate the presence of an exogenous agent. The most common by
far is ethanol, but methanol, ethylene glycol, acetone and isopropyl
alcohol can occasionally be present in sufficient quantities to produce
an increased osmolar gap. Importantly significant toxicities,
particularly from ethylene glycol, can occur with a normal osmolar gap
as the toxic concentrations are quite low. With the passing of time
from ingestion of these substances the osmolar gap falls as the anion
gap rises due to conversion to negatively charged substances."
I was surprised by this suggestion of inadequate sensitivity, but it is
consistent with Roger Bertholf's earlier posting. Does anyone have more
information on whether we might be missing patients we ought to be
treating?
I'd be interested to know from laboratories which use a more specific
analytical approach how quickly you get the required information in
practice.
Jonathan
On Thursday, Jun 5, 2003, at 21:51 Europe/London, David Brown wrote:
> Can someone put me straight on the formula for
> measuring/calculating osmolar gap? Is it the
> difference between the "calculated" and measured
> osmolality? Is there now a reference/standard for
> calculating osmolality?
> It think it might be OK to "suggest" the probability
> of an alcohol ingestion in the light of a significant
> "osmolar gap", but.............
>
> David Brown
>
>
> --- Jonathan Kay <[log in to unmask]> wrote:
>> Seems to work OK for us. In practice clinicians
>> request "ethanol" and
>> we measure the osmolar gap... they don't request
>> "osmolar gap".
>>
>> We refer requests for individual alcohols to a
>> toxicology unit.
>> Occasionally we have to remind clinicians that it
>> may be necessary to
>> treat before the results of assaying individual
>> alcohols are available
>>
>> A and E handbooks should have guidance on ethanol,
>> methanol and
>> ethylene glycol poisoining, written in collaboration
>> with the lab... I
>> think it is important the timing aspects are in the
>> protocol.
>>
>> Methanol poisoning is very rare in the UK compared
>> to the USA...
>>
>> Was this point covered in the Annals review of
>> toxicology requirements?
>>
>> Jonathan
>>
>>
>> On Thursday, Jun 5, 2003, at 10:30 Europe/London,
>> Borland, Bill wrote:
>>
>>> I would be interested to hear any views on the use
>> of the Osmolal Gap
>>> as a screening test for Ethanol. (Coakley at al,
>> Pathology, 1983,15,
>>> 321)
>>>
>>> One of the A&E departments within our Trust uses
>> the Osmolal Gap as a
>>> 'screening' test for alcohol and in only a few
>> clinical situations do
>>> they require a more specific assay for ethanol.
>> They have used the Gap
>>> in this way for many years and I suspect were
>> encouraged to do this by
>>> the lab in the days when it was easier to measure
>> serum osmolality
>>> than ethanol, especially out of hours.
>>>
>>> The danger is that junior medical staff may not be
>> aware of the
>>> limitations of this approach and delay
>> identification of a possible
>>> methanol or ethylene glycol poisoning. Should we
>> be discouraging them?
>>>
>>> It would be useful to find out the practice in
>> other centres.
>>>
>>> William Borland
>>> Principal Biochemist (Toxicology)
>>> Biochemistry Department
>>> North Glasgow NHS Trust
>>> Gartnavel General Hospital
>>> Glasgow G12 0YN
>>> Tel 0141 211 3343
>>> Fax 0141 211 3452
>>> Email bill.borland.wg@northglasgow .scot.nhs.uk
>>>
>>>
>>
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