The Watson-Schwartz reaction is used as a simple screening test for the presence of elevated urinary PBG levels.


2 grams of 4-dimethylaminobenzaldehyde made up to 100 ml with 6N HCl.


Mix 1 ml of freshly-voided urine with 1 ml of reagent. Shake and allow to stand. A pink or red colour implies the presence of either PBG or urobilinogen. Now add 2 ml of chloroform, shake well and allow to settle. Two layers result. PBG, if present, will remain in the top or aqueous layer, imparting a pink colour to it.

It is essential that the pink colour is confined to only one layer: if large amounts of either PBG or urobilinogen are present, both layers will remain coloured. In this case, use a pipette to suck off the upper, aqueous layer, deposit this into a fresh tube, add more chloroform and allow to separate once more.

Interpretation of the Test

Urobilinogen will extract into the lower layer, which contains the chloroform. Thus, if the colour is confined to the lower layer, the urine contains urobilinogen: this is of no significance in terms of porphyria.

Porphobilinogen will extract into the upper, aqueous layer. If this layer is red or pink, PBG is probably elevated which suggests an acute porphyria.


Watson-Schwartz test for PBG

The Watson-Schwartz test

Urine is placed in a test tube (A).

An equal volume of Ehrlich's aldehyde is added. A weakly positive reaction is denoted by a rose-pink colour and a strongly positive reaction by a red colour (B).

Chloroform is added, the tube is mixed and allowed to settle. Chloroform and water are immiscible and the chloroform, being heavier, sinks to the bottom. If the red colour remains in the top, aqueous phase, this confirms the presence of PBG (C).

The Watson-Schwartz reaction is negative in quiescent VP. Any positive result in VP therefore confirms an acute attack. In AIP, the test may remain positive in remission. Absolute confirmation of the presence of an acute attack can only be gained by performing accurate ALA and PBG quantitation in the laboratory and comparing them with previous values for the same patient. Therefore it is suggested that any patient known to have porphyria and who shows a positive Watson-Schwartz reaction, and has compatible clinical features, must be considered to be suffering from the acute attack.

This test is at best a rough guide to a patient's clinical state: in particular, the intensity of the response is very dependent on the degree of concentration or dilution of the urine