Add 0.2ml of sterile saline  to a screw-cap tube

Use a sterile loop/spade to  emulsify two loopfuls/spadefuls of a 4-week old culture in the saline

Add 2ml of the NaNo₃ substrate

Shake well and incubate  upright in a 37EC  water bath for 3 hours and remove

Use the tip of a spatula to  add a small amount of crystalline reagent to the test solution.  The quantity of reagent is not critical

Examine immediately for a  pink to red colour and compare to colour standard