Vitros users: Make sure that you are not measuring CSF protein on sodium fluoride/EDTA preserved samples (eg Teklab yellow cap tubes code FE F2130). Positive bias of several hundred mg/L observed on samples with < 100 mg/L protein. There is nothing in the method file from Ortho about this or other anticoagulants. We have found that there is a substantial positive bias from fluoride/oxalate but were really caught out by this one.
Hands up those who knew that yellow cap tubes COULD contain fluoride/edta. Unfortunately none of us in the lab here did and all thought it was sodium fluoride/potassium oxalate anticoagulant we were using. Someone in the haematology department ordering the blood tubes must have made a mistake on an order. The Teklab paediatric fluoride/oxalate tubes are also yellow caps the only difference being in the code printed on the label.
Has anyone any ideas why the Vitros method has such a strong positive bias on the external QA. ?? really matrix effect ?? I have just diluted 11 fresh serum samples with saline and analysed as CSF. I find reasonable linearity but a bias of +26% compared to the expected value, very similar to the external QC bias and that obtained by analysis of the samples with a pyrogallol red method.
Paul Eldridge
Clinical Biochemistry
University Hospital Lewisham
London SE13 6LH
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