Strengths and limitations
The main strength is the large cohort of pregnancies analysed (612
patients), which to our knowledge is the largest sample of healthy
pregnant women that has been examined for TSBA so far.
We believe the strengths also include the prospective design, the
accurate selection of eligible patients, the generalizability, and the
clinical relevance of the results. Since no correlation has been
reported between TSBA levels and gestational age within the third
trimester26, our results could be applicable to the
third trimester of pregnancy. The duration of the study over one year
allowed us to evaluate the seasonality of TSBA levels.
Finally, all blood samples were analysed in the same laboratory and were
taken ad hoc for the study purposes.
The low variability of ethnicity was due both to the majority of
Caucasian patients in our hospital and to the difficulty of collecting
informed consent from non-Italian speakers. This limited our ability to
evaluate variations in normal TSBA levels between different ethnic
groups.
Even though widely used in clinical laboratories, the most important
limitation of the enzymatic method is low sensitivity, as the lowest
concentration of TSBA measurable is around 1.5
µmol/L18, 29. Nevertheless, 100% of the measured
values resulted above the evaluation limit.
Another limitation is that common hospital food was provided, not
standardized for macronutrients and potentially different from each
other.
Interpretation
Evidence that normal pregnancy may be associated with a mild
sub-cholestatic state has been described in early studies. Although
based on a limited number of patients, an increase has been demonstrated
in the mean of single and total BA concentration in uncomplicated
pregnancies with no other evidence of ICP13-16.
Our results are consistent with early observations that pregnancy is a
sub-cholestatic state13-16, and this can be explained
by the cholestatic effect of reproductive hormones. Normal pregnancy is
an hyperestrogenic state and therefore is associated with a
physiological elevation of TSBA. Thus, obstetricians need to be aware
that healthy pregnant women have increased levels of TSBA when assessing
women who may have ICP.
Early studies described that ICP have a seasonal pattern with increased
incidence in some countries during winter months, suggesting a possible
association with an environmental trigger. This pattern, however, was
not yet demonstrated in clinical studies. Two possible explanations are
low levels of natural selenium and of vitamin D during the winter, as
both deficiencies have been reported in women with
ICP1,5,19,20.
It has recently been suggested that higher thresholds should be used for
the diagnosis of ICP, also given the low risk of stillbirth demonstrated
recently for TSBA levels < 100 µmol/L26, 29.
The assessment of diagnostic criteria for ICP is complex, as the classic
methodologies used to establish diagnostic thresholds for other
pathologies are difficult to apply here. Currently, in clinical
practice, ICP is diagnosed by elevated TSBA levels that are measured
after the insurgence of pruritus, a non-specific symptom that is not
necessarily associated with ICP. Thus, the diagnosis is based on TSBA
alterations (as pruritus without TSBA elevation will not be diagnosed as
ICP) by using non-pregnant reference ranges. In non-pregnant patients
TSBA levels are used as a biomarker for hepatic injury, while in
pregnant patients, the outcome of interest is not liver dysfunction but
rather the absolute elevation in circulating TSBA
levels4. Beside pruritus, TSBA elevation is not
associated with adverse outcome unless reaching a severe ICP
(> 100 µmol/L for stillbirth and > 40 µmol/L
for other adverse outcomes), thereby the diagnostic threshold cannot be
tested by maternal or neonatal outcomes either.
For these reasons, an important limitation when studying the normal
distribution of TSBA levels in pregnancy is that the accuracy
(sensibility and specificity) of the upper limit value of normality
among the ICP population cannot be calculated, as ICP was originally
diagnosed by the same criteria that needs to be tested.
Our results demonstrate that the non-pregnant reference range cannot be
used in the pregnant population. In accordance with the RCOG clinical
guidelines for ICP25, we suggest using in clinical
practice the upper limit value of our normal pregnancy-specific refence
range. Specifically, 14 µmol/L for fasting TSBA values and 20 µmol/L for
postprandial TSBA values. A similar threshold (19 µmol/L) was suggested
by a recent study that re-evaluated the diagnostic thresholds for ICP.
Although using different methodologies, our postprandial threshold was
similar to the random threshold described by Mitchell et
al26. In accordance, we also suggest that patients
with otherwise unexplained pruritus with TSBA levels below the proposed
thresholds should repeat the TSBA measurements since pruritus often
precedes an elevation in TSBA levels2,34.
There is ongoing debate regarding which value should be used in clinical
practice to diagnose ICP.
Postprandial TSBA assessment may be a more sensitive test, whereas
elevated fasting TSBA levels are a more specific indicator of severe
liver disease11,25. Some authors suggest measuring
TSBA levels in the fasting state due to the large overlap between normal
non-fasting values and commonly used thresholds for
diagnosis29. On the other hand, considering that the
adverse perinatal outcomes of ICP are associated with peak TSBA
concentration, a non-fasting measurement has greater clinical
relevance26.
We suggest using both values of TSBA. We recommend distinguishing the
fasting and postprandial status and avoiding random samplings due to
different reference ranges and upper limit values that should be
applied. Fasting TSBA levels may be the most specific for the diagnosis
of ICP and therefore better for confirming ICP. Fasting measurements are
also more predictable, have less variability, and correlate better with
certain risk factors. The postprandial measurement, on the other hand,
is essential for risk stratification. We suggest using postprandial TSBA
levels to assess the severity of the disease and to follow-up patients
with ICP for subsequent management of the pregnancy and eventual active
management.