ABC of UPT :--beta hCG:--The limitations of Urine
for preg tets ?
Most of the
pregnancy tests available to test for the presence of a pregnancy look for the presence of the beta
subunit of hCG or
human chorionic gonadotropin in
the blood or urine. We know that , We also know that human chorionic
gonadotropin is a glycoprotein
hormone secreted by the developing
placenta shortly after implantation
. hCG can be
detected in the urine and serum of pregnant women as early as 6-15 days after conception.. However, in rare cases,
it may be produced by a choriocarcinoma
or some other germ cell tumours
, or even other forms of cancer, e.g. lung cancer.
Urine for pregancy Tests ----how accurate?? Better followed by blood tests in a lab at a later date. False +ve UPT is not
impossible in rare cases.
Home pregnancy tests are available as kits and though
detect pregnancy fairly accurately ;
they should always be followed with blood tests in a lab. Home
pregnancy tests work by detecting raised levels of the
hormone hCG in the urine . To use a home pregnancy urine test
, one must collect urine in a container and dip the test strip into
it. Some tests are available in a solution form and the urine drops
have to be added to it. After a
waiting time interval
the results can be read according to the instructions given .
Rapid hCG Test Card
Rapid
hCG test card is used to obtain a visual , qualitative
result and is intended by
professional and laboratory use only.
The hCG test card is a rapid test to detect the presence of hCG
in urine specimens in a qualitative format sensitive to 10 mIU hCG/ml.
The test utilizes a combination
of monoclonal and polyclonal antibodies
reagents to selectively defect elevated
level of hCG in urine. The immunological specificity of the test card virtually eliminates cross reactivity, interferences from the
structurally related glycoprotein hormones
FSH,LH,TSH at physiological levels.
Blood Tests
A
suspected pregnancy can be confirmed
with the help of a blood test , which
also measures the amount of hCG in
the blood and confirms the pregnancy. A blood test is more reliable
than a urine test, so both are
routinely done to confirm conception.
Pregnancy Tests
Sensitivity and hCG Values
The
pregnancy tests available differ widely in their sensitivity in picking up the pregnancy hormone, hCG .
As a
general rule , hCG levels , if one is pregnant
are between 5-50
mIU a week before the period is due and the hCG levels should
double every two to three days.
Individualization
is important: No rule of thumb about level
of hCG when becomes positive. Not every placenta secrete the hCg at same
quantum. However, it is important to
remember that every woman is different and the time it takes for the fertilized egg to implant in the uterus wall can vary.
When does implanting occurs??
Ans:-A common
misconception is that implantation occurs 7 days after conception , but the research
findings showed that first appearance
of HCG occurred 6-12 days after
ovulation, with 84%
of the pregnancies implanting on
days 8-10 after ovulation. Hence
if one does not get a positive
pregnancy test, it doesn’t necessarily
mean that the subject is not pregnant , because it is
possible that the subject
ovulated later than expected or implantation took longer than the
average.
Brands of HPT
kits :--Various
brands of HPT kits are available and are different
in sensitiveness . Many HPTs claim to be 99% accurate
on the day one menst ( the period)
but research suggests
that most HPTs do not consistently spot pregnancy that early
and even when they do the results
are often very faint.
Brands
do vary:--In a study
done in 2004, researchers tested
the accuracy of different brands
of HPTs and found that only one
brand consistently detected the low levels of hCG
usually present on the first day
of the missed period . The
other tests missed up to 85% of
pregnancies on the first day of the missed period , however most tests
accurately confirmed
pregnancies one week after the missed period.
Pregnancy hCG
measurement Tests : Advantages of
Quantitative Tests
Qualitative
blood tests generally
have a threshold of 25 mIU/mL and are less
sensitive. However
quantitative blood tests can detect hCG levels as low as 1 mIU/mL, while urine tests
have published detection
thresholds between 20 and 100 mIU/ml . Quantitative beta hCG
readings are also used in evaluation of trophoblastic and
other germ cell tumors.
Quantitative
serum beta hCG levels are measured in
mIU/mL and a level above 25 mIU/mL
usually indicates the presence of a pregnancy .Two tests are usually
necessary to verify a normal
increase usually 2-3 days
apart. The increase in
serum hCG can be interactively monitored .
Doubling time varies:--Below an hCG
level of 1,200 mIU/mL the hCG
usually doubles every 48-72 hours, though a rise
of 50-60 % is still considered
normal . Between 1,200 and 6,000
mIU/mL serum , The hCG usually takes 72 -96 hours to double and above 6,000 mIU/mL
the hCG often takes more than
four days to double. Failure to
increase normally may indicate that the pregnancy is not developing well and
can be an early sign for a
possible miscarriage or an ectopic pregnancy . After a miscarriage.
HCG levels
fall steadily back to the non
pregnancy range . Also
an ectopic pregnancy may be
suspected when hCG levels
fall to double particularly if
the HCG level rises, fails and
rises again . Home pregnancy tests : Drug
interactions
Most medicines
over the counter and prescription including
birth control pills and antibiotics
does not affect the results of a home
pregnancy test. Alcohol and
illegal drugs do not
affect HPT results. Only
medicines that have the pregancy hormone hCG , such
as once used to treat infertility
can give a false positive
test result.
False Positive
and False negative Tests
False negative
readings can occur when testing is done
too early and earlier the test is
performed the higher the chance of a
false negative results. A false
negative result can also stem
from using a diluted urine sample
apart from taking a test too early in pregnancy. First morning urine
contains the most concentrated presence
of hCG which makes it the ideal
sample for pregnancy testing .
False positive
tests are rare through there are
instances and conditions where
they can occur. In case some tissues
in a non pregnant women produce hCG it may give a false
positive test. False positive
results can result from diseases like
chorio carcinomas. IgA deficiencies
hetero plastic diseases gestational
trophoblastic neoplasms and testicular
germ cell malignancies .
Secondly research indicates
that half of all conception
do not go forward to develop as pregnancy
. and may abort. A false positive
can be seen in this situation.
Medications that can cause a false positive
are the fertility medications
that contain hCG. However fertility drugs and medications that do not contain hCG will not precipitate false positive as well . A percentage
of false positive pregnancy
tests can also be
attributed to misinterpretation of results
due to a failure to follow
test instructions with
precision. Additionally at the same gestational stages women produce quantitatively different levels of hCG . Hence the first indication of a positive
result on a home pregnancy test may vary between women despite similar gestational stage.
Evaporation
line : Moreover many home pregnancy
tests show a positive
or unclear result when read
well after the suggested 3-5
minute window; independent of an actual
pregnancy. This type of false positive
is known as an evaporation line.
Phantom hCG:
Some individuals react to some substrate in the
test and thus will display a consistently low positive
blood pregnancy test even though
they are not pregnant. This phantom
hCG may lead to serious misdiagnosis
and intervention, but can be
detected with serial dilutions. Patients with phantom hCG have a positive blood hCg but a negative urine hcG test
Need for patience in
Pregnancy tests
Since pregnancy tests look for the hCG
hormone which is produced
once the fertilized egg has
implanted in the uterine wall in most
cases, this happens about 6 days after
conception. But studies show
that in up to 10 percent of women , the embryo doesn’t implant
until much later , after the first
day of the missed period. Hence home pregnancy tests will be accurate as soon as one day after a
missed period for some women but not for others. And because the amount of hCg in the urine
at different points in early
pregnancy is different for every woman Some woman will have
accurate results on the day of
the missed period while others will need
to wait longer. But whatever
may be time period a positive
pregnancy test further
confirmed to be healthy by
ultrasound techniques makes all the hard work and effort
of planning pregnancy worthwhile
Role of
ultrasound in pregnancy detection
Once a pregnancy
has advanced past the first 6-8
weeks the pregnancy is usually
easier to follow by ultrasound as
more information is
obtained in real time. In general a
pregancy is detectable 25 days after
ovulation by transvaginal sonography
usually corresponding to an
hCG level of >1,500 miu/mL
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