In order to expose substituted, cheated and faked urine specimens submitted for a drug test, a simple and highly sensitive screening method has been developed for the detection of urea in the specimens. This method uses the coloration of a piece of pH test paper which is wetted and set into the headspace of a sample vial containing “urine”, by absorbing NH₃ gas generated by the urease reaction. The present method named, “ U rease- H ead s pace method” (UHS method), was evaluated by applying it to various diluted or adulterated urine samples. The detection limit of urea in water was 2×10⁻⁴%, which was 100 times higher sensitivity compared with a conventional p -(dimethylamino)cinnamaldehyde (DAC) test. The UHS method was applicable even to deeply colored specimens such as bloody urine because the coloration occurs in the headspace of the sample vial. The UHS method quickly revealed the substituted specimens, e.g. water and green tea. Thus, the present UHS method will be effective for the validity determination of urine specimens, which is increasingly crucial in forensic drug examination.