Methaemoglobin is occasionally produced therapeutically and is probably seen more commonly from recreational amyl nitrate use. There is a wide range of drugs and chemicals that can cause the oxidative stress that transforms Fe2+ to Fe3+ causing the patient to appear cyanosed, altering the oxygen carrying capacity and turning blood to a chocolate-brown colour.
Traditonally best side tests have been based on the detection of chocolate brown blood on tissue paper such as this example of a patient with propanil poisoning.
A refinement and validation of this observation using a color chart has been published: Shihana F, et al. A simple quantitative bedside test to determine methemoglobin. Ann Emerg Med 2010 Feb;55(2):184-9 PMID19818531 Fulltext
The colour chart instructions and the data for constructing it can be downloaded here [93 KB]
A computer generated chart for printing can be downloaded here [121 KB]