Cyanide/nitroprusside (CNP) test for cystine/homocystine

• To 1 ml urine add 2 drops concentrated ammonia solution followed by 0 5 ml of 10% (w/v) sodium cyanide (care needed).
• Leave for at least five minutes and then add 5 drops of 5% (w/v) sodium nitroprusside.
• Positive samples give a distinct magenta colour.
• it is advisable to run a positive control in parallel.
• If an authentic urine is not available a normal urine spiked with cystine (0 3 mmol/l) is suitable; aliquots can be stored at - 20°C.
• Any compound that contains sulphydryl groups or yields sulphydryl groups on reduction
• will give a positive reaction.
• This test is sensitive to 0-25 mmol/l cystine and 0-25 mmol/l homocystine.
• Concentrated- normal urine samples (creatinine greater than 7 mmol/l) may give a misleading positive result.

A simple test for the determination of cystinuria and of aminoaciduria is described.

• the basis of the test is the color reaction between nitroprusside and thiol groups.
• The reagent is a dry mixture to which the urine samples are applied. The ease of
• peformance of the test makes it ideal for use as a screening procedure.
• The reaction is nonspecific, and positive tests require further investigation by more elaborate laboratory procedures.