ABSTRACT
The technical and diagnostic performance of the Amerlite-MAB enhanced luminescence assay for free thyroxine (FT4) was assessed in a multicenter evaluation trial. The euthyroid central 95% reference range for FT4 (1393 subjects) was 11.5-27.7 pmol/L (by cumulative frequency plot with two-tailed 2.5% cutoffs). Results (y) agreed with those of similar radioactive method (Amerlex-MAB FT4) (x): y = 1.06 x + 0.54, Sylx = 335.5, n = 235). Mean within-assay precision (CV) in six centers was 5.7% at 6 pmol/L and 2.6% at 51 pmol/L FT4; between-assay precision (CV) was 7.2% and 3.6%, respectively. Diagnostic performance was assessed in all patient groups usually encountered, including those with nonthyroidal illness and extreme binding-protein anomalies. In elderly euthyroid subjects, the proportion of above-normal FT4 values exceeded that in younger age-groups. These increases often accompanied normal estimates of thyrotropin and, in some cases, might have arisen by interference from medication that is taken more often by the elderly.
Subject(s)
Immunoenzyme Techniques , Luminescent Measurements , Thyroxine/blood , Adolescent , Adult , Aged , Aging , Blood Proteins/metabolism , Child , Child, Preschool , Female , Humans , Hyperthyroidism/blood , Hypothyroidism/blood , Immunoenzyme Techniques/statistics & numerical data , Infant , Male , Middle Aged , Pregnancy , Quality Control , Reference Values , Sensitivity and Specificity , Sex CharacteristicsABSTRACT
We describe the development and validation of a one-step nonradioactive immunoassay for free thyroxine (Amerlite-MAB FT4) in serum or plasma, in the dedicated Amerlite enhanced luminescence assay system. A monoclonal antibody to thyroxine (T4), conjugated with horseradish peroxidase (HRP; EC 1.11.1.7), competes for binding with FT4 or with a conjugate of a protein and L-triiodothyronine immobilized onto the surface of the assay microwells. The signal is generated by antibody-HRP bound to the protein-T3 conjugate, with luminol as substrate. The assay design exploits the high sensitivity of luminescence signal detection, permitting minimal sample dilution and T4 sampling by the antibody. It withstands progressive dilution of serum and is unaffected by T4-binding proteins in serum. The disclosure and validation of this FT4 assay follows guidelines recommended by the American Thyroid Association.
Subject(s)
Luminescent Measurements , Thyroxine/blood , Antibodies, Monoclonal , Edetic Acid , Fatty Acids, Nonesterified/blood , Humans , Immunoenzyme Techniques/statistics & numerical data , Kinetics , Plasma/chemistry , Prealbumin/metabolism , Quality Control , Sensitivity and Specificity , Serum Albumin/metabolism , Thyroxine-Binding Proteins/metabolismABSTRACT
We describe a one-step, labeled-antibody radioassay for measuring free thyroxin (FT4) in serum or plasma, based on a novel principle. FT4 in the sample competes with a gross molar excess (over antibody) of a cross-reactant (L-triiodothyronine, T3), chemically coupled to magnetizable polymer particles, for binding to avid 125l-labeled monoclonal anti-thyroxin antibodies. As in conventional immunoassays, 125I counts bound to the solid phase (T3-magnetizable particles) are inversely proportional to sample FT4 concentration. We demonstrate here the development and technical validity of this new method.
Subject(s)
Radioimmunoassay/methods , Thyroxine/blood , Antibodies, Monoclonal , Antibody Affinity , Humans , Iodine Radioisotopes , Magnetics , Microspheres , Serum Albumin/metabolism , Thyroxine/immunology , Thyroxine-Binding Proteins/metabolism , Triiodothyronine/blood , Triiodothyronine/immunologyABSTRACT
The technical and diagnostic performance of a labeled antibody radioassay for free thyroxin (Amerlex-MAB* FT4) was examined in a multicenter trial. The overall mean within-assay precision (CV) in eight centers was 6.2% at 30 pmol/L and 3.6% at 13 pmol/L concentrations of FT4. Between-assay precision was 5.4% at 18 pmol/L and 6.8% at 50 pmol/L. The euthyroid central 95% reference range for FT4 was 11.3 to 24.3 pmol/L. Results of the method correlated well with those of an analog radioimmunoassay: [Amerlex-MAB* FT4] = 1.09 [Amerlex-M FT4] + 1.50 pmol/L (r = 0.96, n = 732). Clinical performance of the assay was better than that of first-generation analog tracer assays for sera from patients with nonthyroidal illness or binding-protein abnormalities.
Subject(s)
Radioimmunoassay/methods , Thyroxine/blood , Autoantibodies/blood , Female , Humans , Hyperthyroidism/blood , Hypothyroidism/blood , Pregnancy , Radioimmunoassay/standards , Radioimmunoassay/statistics & numerical data , Reference Values , Thyroxine/immunology , Thyroxine/therapeutic use , Thyroxine-Binding Proteins/metabolism , Triiodothyronine/immunologyABSTRACT
Free thyroxin (FT4) estimates by two immunoassays were compared with the concentrations of albumin in serum of apparently euthyroid subjects who either were (n = 99) or were not (n = 327) suffering from severe nonthyroidal illness (sNTI). In neither group was FT4 significantly correlated with albumin (P greater than 0.05), according to a "labeled antibody" radioassay (Amerlex-MAB). On amalgamating both groups, correlation with albumin was positive and significant (P less than 0.001). In the group with sNTI, both FT4 and albumin concentrations were decreased (mean FT4 to 77% and mean albumin to 61% of the respective reference means). For an analog radioimmunoassay (Amerlex-M), FT4 in all groups was significantly (P less than 0.001) correlated with albumin. Correlation coefficients were greater than with Amerlex-MAB for both sNTI and euthyroid groups, as well as for the joint panel. Mean FT4 in sNTI was only 44% of the reference mean. Lower radio-tracer "analog" values in sNTI are exaggerated by additional technical artefacts resulting from tracer binding to albumin.
Subject(s)
Health Status , Serum Albumin/analysis , Thyroid Gland/physiology , Thyroxine/blood , Adolescent , Adult , Age Factors , Aged , Female , Humans , Male , Middle Aged , Radioimmunoassay , Reproducibility of Results , Thyrotropin/bloodABSTRACT
Free thyroxin (FT4) concentrations, total thyroxin/thyroxin-binding globulin (T4/TBG) ratios, and thyrotropin (TSH) and albumin concentrations were measured in serum in a longitudinal study in each of the three trimesters of 25 normal pregnancies. In late pregnancy, FT4 estimates by assays reputedly either affected or unaffected by albumin were in the lower half of the reference range for nonpregnant subjects. T4/TBG ratios and albumin concentrations were similarly lower. FT4 overall was significantly (P less than 0.001) correlated with these latter two values. Serum TSH concentrations increased as FT4 declined in late pregnancy. Nonesterified fatty acid (NEFA) concentrations were too low to displace T4 from its binding proteins and were not correlated with other measurements. Within any one of the trimesters, FT4 and T4/TBG were independent of variations in TBG or albumin concentrations. This implies that lower FT4 concentrations in late pregnancy are real, merely coinciding with parallel decreases in albumin. They are not artefacts of albumin-affected assays.
