SIMPLE SCREENING TEST
FOR THE ACUTE ATTACK
The Watson-Schwartz reaction is used as
a simple screening test for the presence
of elevated urinary PBG levels.
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
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
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.
will extract into the upper, aqueous
layer. If this layer is red or pink,
PBG is probably elevated which suggests
an acute porphyria.
is placed in a test tube (A).
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).
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.