The measurement of proteinuria
is no less problematic. The urinary protein excreted
in the condition pre-eclampsia arises as a result of
glomerula endotheliosis. However, it also indicates
a generalised increase in capillary permeability in
other organ systems of the body ( 1).
The presence of significant proteinuria does act as
a marker for the severity of the disease pre-eclampsia
and patients who suffer from hypertension and proteinuria
are at an increased risk of small for gestational age
fetuses and perinatal mortality as well as maternal
morbidity (2).
The most commonly used screening method is dipstix
testing of a random urine sample. These however are
notoriously inaccurate. Several published papers have
shown a poor positive predictive value for the presence
of protein as identified by the dipstix as well as
high false negative rates. One plus (+) of protein
on the dipstix is said to approximate to 300 mgs of
protein loss in a 24 hour urine collection. This level
is the upper limit of normal in pregnancy. However
the measure depends upon the concentration of the urine,
which will fluctuate with fluid in take and the timing
of the urine specimen. Two pluses (++) on a dipstick
is more secure when making the diagnosis of significant
proteinuria.
However, all dipstick testing needs to be confirmed
with 24 hour urine collections which are currently
accepted as the "gold standard" for quantification
of urinary protein loss. As we will see later, these
too have their problems.
Automated analysis eliminates observer variability
associated with the dipstix and it has been shown to
increase the sensitivity for the measurement of proteinuria
(3).
The spot urine protein: creatinine ratio, which is
a newer method for quantifying protein loss, correlates
well with a 24 hour urine collection and has several
advantages. This technique improves detection of proteinuria
and allows for the concentration of the urine in quantifying
protein loss. The results are evidently quicker than
the 24 hour urine collection which is inconvenient
and difficult particularly for some women.
Despite the difficulties in quantifying proteinuria
in pregnancy when the disease process of pre-eclampsia
is viewed in the round, it is a useful marker of severity.
It is incumbent upon clinicians to recognise the deficiencies
in the measurement techniques, for proteinuria and
blood pressure; and ensure that all borderline measurements
are repeated in order that women are not wrongly labelled
with the diagnosis of pre-eclampsia. It should also
be noted that the disease may present in different
ways without significant proteinuria. This issue will
be raised in the next section on the definitions and
classification of the hypertensive disorders in pregnancy.
References
1. Brown MA. Capillary permeability and extra cellular fluid volumes in pregnancy-induced
hypertension. Clin Sci 1989;77:599-604, Abstract
2. Halligan AW. Dipstick proteinuria: caveat emptor. Br J Obstet Gynecol 1999;106:1113-5, Abstract
3. Saundan PJ. Improved methods of assessing proteinuria in hypertensive pregnancy.
1997;104:1159-64, Abstract
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