Anti-TPO Testing Procedure
Laboratory Method and Assay Selection
Anti-TPO antibodies should be measured using quantitative immunoassays (ELISA or RIA) rather than older immunofluorescence or passive hemagglutination methods, as these modern assays provide superior sensitivity and specificity for autoimmune thyroid disease diagnosis. 1, 2
Recommended Testing Methodology
Use monoclonal antibody-assisted RIA or ELISA methods that employ purified thyroid peroxidase antigen, as these provide the highest sensitivity (96% for Hashimoto's thyroiditis) and specificity (100%) compared to traditional microsomal antibody tests 1, 2
The anti-TPO assay has replaced the older thyroid microsomal antibody (anti-M Ab) test because it is more sensitive, specific, and uses only tracer amounts of purified antigen 1
Modern anti-TPO assays are unaffected by circulating thyroglobulin concentrations up to >10,000 ng/mL, eliminating a major source of interference seen with older methods 1
Sample Collection and Handling
No special timing requirements exist for sample collection, though testing should ideally be performed when the patient is not acutely ill 3
Store samples appropriately according to laboratory protocol until analysis 2
Interpretation of Results
Diagnostic Thresholds
Normal range: Most healthy individuals have anti-TPO levels <52 U/mL, with the upper limit of normal typically set at 50-100 U/mL depending on the assay 4
Clearly elevated: Anti-TPO >200-500 U/mL is highly specific for autoimmune thyroid disease 2, 4
Positive results: 88-99% of patients with Hashimoto's thyroiditis have elevated anti-TPO antibodies, compared to 53-74% of patients with Graves' disease 1, 2
Clinical Significance by Disease
Hashimoto's thyroiditis/autoimmune hypothyroidism: 96% sensitivity with anti-TPO >200 U/mL cutoff, with levels ranging from 11-90,000 U/mL 1, 2
Graves' disease: 53-74% positivity rate, with generally lower antibody levels than Hashimoto's 1, 2
Normal controls: Only 8.4% have detectable anti-TPO antibodies (range 11-210 U/mL), establishing the test's high specificity 1
Non-thyroidal illness: Mean anti-TPO levels of 39 ± 34 U/mL, with no clearly elevated values (>500 U/mL) 2
Important Testing Considerations
Advantages Over Older Methods
Anti-TPO assays avoid false-positive results seen with microsomal antibody tests in patients with high anti-thyroglobulin antibodies or non-thyroid autoimmune diseases (such as mitochondrial antibodies) 2, 4
The correlation between anti-TPO RIA and traditional passive hemagglutination is high (r=0.979), but discrepancies occur almost exclusively in sera with negative or low microsomal antibody titers, where anti-TPO demonstrates superior sensitivity 1
Concurrent Testing Recommendations
Always measure anti-thyroglobulin antibodies when checking serum thyroglobulin levels in thyroid cancer patients, as TgAb can interfere with thyroglobulin measurement and mask true levels 5
Consider measuring TSH, free T4, and T3 simultaneously with anti-TPO to assess current thyroid function status, as significant correlations exist between anti-TPO positivity and abnormal thyroid function tests (p<0.0001) 3
Clinical Pitfalls to Avoid
Do not rely on anti-TPO antibodies alone to diagnose thyroid dysfunction—thyroid function tests (TSH, free T4) are required to determine if hypothyroidism or hyperthyroidism is present 6
Recognize that 8-11% of normal individuals may have low-level positive anti-TPO antibodies without clinical disease, requiring correlation with thyroid function tests and clinical presentation 1, 5
Be aware that different laboratory platforms produce varying results, making direct comparison of absolute values across laboratories problematic 5
Understand that anti-TPO antibody levels can fluctuate with treatment: they decrease by >50% in Graves' disease patients achieving euthyroid state with methimazole, and show variable changes in hypothyroid patients on levothyroxine replacement 2