Summary And Explanation - Siemens IMMULITE 2000 TBG Manual Del Usuario

IMMULITE
English
Intended Use: For in vitro diagnostic use
with the IMMULITE 2000 Systems
Analyzers — for the quantitative
measurement of thyroxine-binding globulin
(TBG) in serum, as an aid in the diagnosis
and treatment of thyroid diseases.
Catalog Number: L2KTB2 (200 tests)
Test Code: TBG Color: Dark Blue

Summary and Explanation

Thyroxine-binding globulin (TBG) is an
acidic glycoprotein consisting of a single
polypeptide chain with a molecular mass
10,11,12
of 54,000 daltons.
principal carrier proteins for both thyroxine
(T4) and 3,5,3-triiodothyronine (T3); the
other thyroid hormone carrier proteins are
thyroxine-binding prealbumin (TBPA) and
albumin. TBG, though present in
significantly lesser amounts than TBPA
and albumin, has a higher affinity for the
thyroid hormones and is therefore the
chief carrier protein.
under 0.05 percent of total circulating T4 is
present as unbound (free) hormone. The
bound fraction is distributed among the
carrier proteins as follows: TBG, 70–75%;
TBPA, 15–20%; and albumin, 5–10%.
Both bound and unbound T4 are present
in an equilibrium that tends to reassert
itself in the face of altered levels of the
carrier proteins by inducing a
corresponding alteration in the total level
of T4 in circulation, while leaving the free
T4 level relatively unchanged.
free T4 concentration may be expected to
correlate more closely than the total T4
concentration with clinical thyroid status,
for an abnormal total T4 result may signify
either an abnormality in thyroid function or
simply a variation (physiological or
pathological) in the carrier proteins.
Thus, for example, the TBG elevations
typical of pregnancy,
and estrogen therapy will cause the total
T4 level to increase, often beyond the
limits of normal, without inducing a
corresponding elevation in the free T4
5,6
level. Again, alterations in the TBG level
2
®
2000 TBG
It is one of three
1,2
In healthy subjects,
2
2
Hence the
1
4
oral contraceptives
sometimes mask the effects of abnormal
thyroid function by elevating the total T4
level of a hypothyroid patient — or
lowering that of a hyperthyroid patient
into the euthyroid reference range. Here,
too, the free T4 concentration will typically
reflect the patient's actual thyroid status
more reliably than the total T4
concentration.
Historically, estimation of free T4, often via
the free thyroxine index (FT4I), has been
the most frequently used test for thyroid
dysfunction.
typically involves multiplying a total T4
result by a T3-uptake (or T4-uptake)
result. The T3 uptake test yields a relative
measure of the unsaturated binding sites
on TBG, rather than a direct, quantitative
estimate of the concentration of this carrier
protein. The TBG saturation index (TBG-
SI) has been used, under various names,
e.g. thyroid hormone binding ratio (THBR),
as a substitute for the FT4I.
is essentially just the ratio of total T4 to
TBG, multiplied by some factor
(depending on the units) so as to achieve
a standard reporting scale. When total T4
and TBG are both expressed in molar
units as nmol/L and the factor is taken as
100, the TBG-SI represents the fractional
occupation of TBG binding sites by T4,
expressed as a percent.
Principle of the Procedure
IMMULITE 2000 TBG is a solid-phase,
competitive chemiluminescent enzyme
immunoassay.
Incubation Cycles: 1 × 30 minutes
Time to first result: 35 minutes.
Specimen Collection
3
The use of an ultracentrifuge is
recommended to clear lipemic samples.
Hemolyzed samples may indicate
mistreatment of a specimen before receipt
by the laboratory; hence the results should
be interpreted with caution.
Centrifuging serum samples before a
complete clot forms may result in the
presence of fibrin. To prevent erroneous
results due to the presence of fibrin,
ensure that complete clot formation has
IMMULITE 2000 TBG (PIL2KTB-18, 2010-07-07)
7
Calculation of the FT4I
13
The TBG-SI
13
4
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