TABLE 1.

Troubleshooting table.

Problem	Possible reasons	Solution
Color formation from Hcy is not observed at desired detection limit	Hcy solution is not fresh	Use sample containing reduced Hcy immediately
	Low Hcy levels	Run replicates; concentrations of methyl viologen dichloride hydrate and/or buffer might need adjustment (e.g., see ref. 15); time and/or temperature might need adjustment
	Wrong buffer	Buffer must be Tris; no substitutes are acceptable
Solution color fades rapidly	Methyl viologen dichloride hydrate radical cation is quenched	Sample should not be allowed to cool before visual inspection; do not move sample
Solution color formation is nonselective; observed with cysteine and/or other thiols	pH is too high, resulting in nonselective formation of reducing disulfide radical cation	Check buffer solution and check that the pH in the plasma sample is neutral using pH paper
	Metal catalysis	Avoid contact with metals; use deionized water
Solution turns dark greenish after 4 min	Temperature is too high or heating period is too long	Use milder heating
Foaming observed in vial	Leak	Make sure vials are tightly closed before heating