Methaemoglobin (MetHb) forms when the iron component in haemoglobin is oxidised from the ferrous (Fe2+) to the ferric (Fe3+) state. Methemoglobin is unable to bind oxygen and therefore cannot deliver oxygen to the tissues. Increases in methaemoglobin may occur due to genetic disorders in red blood cell metabolism or haemoglobin structure. It can also be acquired by exposure to oxidative drugs, chemicals or toxins e.g. nitrites/nitrates (amyl nitrate, nitroprusside), local anesthetics (benzocaine, lidocaine) and antibiotics (sulphonamides, dapsone).
The characteristic presentation of a patient with significant methaemoglobinaemia is persistent cyanosis despite oyxgenation. The arterial blood may be chocolate brown in colour and remains dark on aeration. The urine may be also be discoloured brown or black.
|Tube||06 (HP) - Heparin No Gel|
|Additional Information||Samples analysed on co-oximeter on blood gas analyser. Whole blood is required in lithium heparin. EDTA samples must NOT be used, as this can damage the gas machine sensors. DO NOT SPIN OR SEPARATE. Sample will be analysed by acute lab staff, notify them on arrival. Sample must be received within 2 hours of collection.
Samples are accepted from GPs providing they reach the laboratory within 2 hours of collection - they need to be tracked through reception and tested on the Core Lab gas machine - pass to a Chemistry BMS.
|Send to||Co-oximetry on blood gas analyser.|
|Collection Con||Balanced (blood gas) heparin or non-gel heparin are the ONLY preservatives that should be used for these samples.|
|Telepath||MTHB1L / MTHB1J|