Screening tests for porphobilinogen are insensitive. The problem and its solution

Standard screening tests for porphobilinogen (PBG) do not turn positive until the concentration of PBG exceeds 10-20 times the upper limit of normal. The authors have developed a screening procedure that uses a new ion-exchange resin to separate PBG from interfering substances in urine. On addition of Ehrlich's reagent, the color is more intense than that produced in the Watson-Schwartz test, and its spectrum more closely resembles that of pure PBG. By measuring the absorbance of this solution at 555 nm, it is possible to discriminate between urine samples with 9 mumol/L (2 mg/L) of PBG and those with no detectable PBG. The new screening test had positive results in two patients with latent acute intermittent porphyria; the Watson-Schwartz test had negative results in both cases. This procedure is easy to perform, has much greater sensitivity than the Watson-Schwartz test, and uses objective spectrophotometric data to separate positive from negative results.