It is suspected that serum gel tube samples may give falsely low readings as some of the phenobarbital may be absorbed into the gel. Current advice is to avoid this type of submission tube.
Can a phenobarbital assay be run on a haemolysed sample?
Ideally all samples submitted should show no signs of haemolysis, however mildly haemolysed samples may be acceptable.
The most useful single sample is the trough phenobarbital concentration, i.e. immediately before a dose is due. If it is not practical to collect a sample at this time then try to take subsequent samples at the same time in the dosing cycle. Some authors advocate the use of peak phenobarbital levels as well (2 to 4 hours post-pill) to check the maximal serum concentration. A recent article (Levitski RE & Trepanier LA, 2000) suggested that in most cases this is not necessary.
- is it OK to submit these?
Yes this is a common finding. Lipaemic samples usually don’t present a problem. They may need to stand for 24 hours after arrival at the laboratory to settle out before they can be tested. Hence there may be a delay in receiving the result.
Please contact one of the veterinary advisors at Vetoquinol, who will be very happy to try to help. Have all the case details to hand when you call to facilitate case discussion.
Different laboratories and books quote reference ranges using a variety of reference units. You should always be careful to check the reference ranges for each individual laboratory.
The references ranges for therapeutic serum phenobarbital
concentrations in the two common units in use are:
• micromol/l (typical reference range 65-194 micromol/l).
• microgram/ml (typical reference range 15-45 microgram/ml).
There are two reasons to provide this information. Firstly we collate the results from all the samples submitted with completed forms and have created a database of this information. We are now investigating the correlation between various parameters - for example the percentage of cases controlled at a certain serum phenobarbital level. Secondly, should you want advice on a specific result this baseline information might be needed to discuss the case.
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