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Sean, The fact that you are seeing this problem with plasma ( but not serum ) makes me wonder if some of these plasma samples contain platelets, which could be lysed during the assay, releasing LD. Do you remove all platelets from plasma with your current combination of g force and centrifugation time ( check platelet count of plasma samples using haematology analyser )? The patient with idiopathic thrombocytopoenic purpura, however, is difficult to explain. How hard would it be omit NADH from your LD assay mixture, and monitor absorbance changes for serum and plasma samples causing you problems? I am sure that Beckman could provide you with the reagent formulation in confidence. This would then allow you to resolve whether the interference was chemical or optical. We had a similar problem many years ago. I have attached the paper summarising our investigations. Hope this helps, and it would be interesting to know what you find. Kind regards, Michael Peake, Flinders Medical Centre, Adelaide, South Australia. _____ From: Clinical biochemistry discussion list [mailto:[log in to unmask]] On Behalf Of Sean Maguire Sent: Thursday, 9 July 2009 11:12 PM To: [log in to unmask] Subject: LDH (pyruvate to lactate) nonlinear reactions Dear colleagues, Here in the Biochemistry lab of the Mater Hospital in Dublin, Ireland, we use the Beckman Coulter DX and we assay LDH by P to L in plasma (Lithium heparin). Absorbances of the reaction can be viewed after the assay is over by highlighting LDH and then ABS. We get the occasional plasma where the absorbance in the reading time is not linear. The reaction , instead of being linear in the read time curls upwards at the end as if NADH was being produced from another source. We dilute with a control (1:1) and then we get linearity and also higher answers for the patient sample which we then report. Serum samples on these same patients with the the interference are linear in the read time. The interference in plasma seems to disappear after about 5 hours which makes it hard to examine. The patients in whom we see this interference are invariably patients with cancer or on cancer treatment (most from Haematolgy OPD). One patient had idiopathic thrombocytopoenic purpura and was apparently on no unusual treatment. We could switch to serum for all LDHs as a solution. Can I get your advice as to where the interference is coming from? No one drug is implicated in all the patients... Many thanks. Dr. Sean Maguire, Principal Biochemist, Mater Hospital, Dublin, Ireland. ------ACB discussion List Information-------- This is an open discussion list for the academic and clinical community working in clinical biochemistry. Please note, archived messages are public and can be viewed via the internet. Views expressed are those of the individual and they are responsible for all message content. ACB Web Site http://www.acb.org.uk List Archives http://www.jiscmail.ac.uk/lists/ACB-CLIN-CHEM-GEN.html List Instructions (How to leave etc.) http://www.jiscmail.ac.uk/ ------ACB discussion List Information-------- This is an open discussion list for the academic and clinical community working in clinical biochemistry. Please note, archived messages are public and can be viewed via the internet. Views expressed are those of the individual and they are responsible for all message content. ACB Web Site http://www.acb.org.uk List Archives http://www.jiscmail.ac.uk/lists/ACB-CLIN-CHEM-GEN.html List Instructions (How to leave etc.) http://www.jiscmail.ac.uk/