Thyroglobulin Antibody and Thyroglobulin, IMA or LC/MS-MS-13509
Test info
Thyroglobulin by LC-MS/MS
If TgAb is negative, serum thyroglobulin is measured by a sensitive IMA.
If TgAb is positive, serum thyroglobulin is measured by an LC-MS/MS method. .
Thyroglobulin (Tg) measurement is intended to aid in monitoring for the presence of orthotropic and/or metastatic thyroid tissues in patients who have had thyroid gland ablation (using thyroid surgery with or without radioactivity). Measurement of thyroglobulin antibody (TgAb) is used to identify patient samples that may be affected by TgAb interference in the measurement of Tg. Quantitative TgAb concentrations can also serve as a surrogate tumor marker for DTC recurrence and for monitoring changes in tumor mass in certain patients.
Any changes in serum Tg concentrations should be interpreted in light of the total clinical presentation of the patient, including clinical history, data from additional testing and other appropriate information. Single measurement of thyroglobulin close to the limit of detection is of minimal value in assessing disease status. Serial determinations are required, and should be referenced to the postsurgical baseline Tg result when possible. Evaluation of increasing Tg levels over time are more clinically important.
Serial thyroglobulin (Tg) and/or thyroglobulin antibody (TgAb) testing on an individual patient should be performed by the same method for reliable interpretation.1,3-5 When a change in Tg method is necessary, it is recommended to reëstablish a new baseline Tg level to then interpret further change over time.
It should be noted that the thyroid tumors of some patients fail to secret a detectable Tg concentration or may secrete abnormal Tg isoforms that are not detectable by some assays used to measure Tg.1 Measurement of preoperative Tg levels can provide the clinician with insights regarding the tumors Tg production and secretion and support the utility of postoperative Tg monitoring.
As with all two-site "sandwich" immunoassays, some of the analytical limitations of Tg IMA include hook effects, human mouse antibody (HAMA), and anti-Tg interference.6 The Access Thyroglobulin assay does not demonstrate any "hook" effect for concentrations up to 40,000 ng/mL. The Access Thyroglobulin Antibody does not demonstrate any "hook" effect up to approximately 50,000 IU/mL. For samples that are Tg antibody positive, Tg will be measured using a sensitive LC/MS-MS method that is not subject to TgAb interference.
Rare amino acid sequence mutations within Tg could potentially cause a false-low result in the Tg LC/MS-MS assay, if the sequence variation occurs within the tryptic peptide measured by the assay or eliminates the tryptic cleavage site.7 In the heterozygote state, the result would be an apparent reduction in Tg concentration by about 50%, while the homozygous state no TG would be detected.
Specimen
This test may exhibit interference when sample is collected from a person who is consuming a supplement with a high dose of biotin (also termed as vitamin B7 or B8, vitamin H, or coenzyme R). It is recommended to ask all patients who may be indicated for this test about biotin supplementation. Patients should be cautioned to stop biotin consumption at least 72 hours prior to the collection of a sample.
x 2Submission of the minimum volume does not allow for repeat testing
- It is suggested to draw two (2) tubes to achieve volume requirements
- Immediately following collection, thoroughly mix sample by gently inverting 5 times
- Allow sample to clot for a minimum of 30 minutes
- Spin within two (2) hours of sample collection
Gold serum separator (SST) tube x2
x 2- It is suggested to draw two (2) tubes to achieve volume requirements
- Immediately following collection, thoroughly mix sample by gently inverting 5 times
- Allow sample to clot for a minimum of 30 minutes
- Spin within two (2) hours of sample collection
- Transfer serum to two (2) Transfer vial/tube with cap - 12mL (LabCorp), labelled as serum
x2Ambient (preferred) - 7 days
Refrigerated - 7 days
Frozen - 14 days
Performance
TgAb: Beckman Coulter immunometric assay
Tg-IMA: Beckman Coulter immunometric assay
Tg: Liquid chromatography/tandem mass spectrometry (LC/MS-MS) lab-developed test. Both Tg assays are calibrated against CRM-57 international standard for Tg as recommended in recent guidance documents.
Clinical and Interpretive info
Thyroglobulin Antibody: 0.0 – 0.9 IU/mL
If NEG: Thyroglobulin by IMA 1.4 – 29.2 ng/mL
If POS: Thyroglobulin by LCMS 1.4 – 29.2 ng/mL