Abstract: A composition, test means (and device) and method for determining peroxidatively active substances in a test sample are disclosed. The composition, test means (and device) and method are rendered resistant to the adverse affects of ascorbate which may be present in the sample by the inclusion in the composition of a metal chelate which is polycarboxyalkylamine derivative having the formula: ##STR1## where: (a) R.sub.1 is hydrogen or straight or branched chain alkyl alcohol or alkyl carboxylic acid radicals having from 2 to 3 carbon atoms; R.sub.2, R.sub.3, R.sub.x and R.sub.y, same or different, are straight or branched chain alkyl alcohol or alkyl carboxylic acid radicals having from 2 to 3 carbon atoms; where at least two of R.sub.1, R.sub.2, R.sub.3, R.sub.x or R.sub.y are alkyl carboxylic acid radicals so defined;(b) R.sub.p and R.sub.