Safeguard your immunoassays with proven blocking agents. This catalog discusses immunoassay blocking agents to reduce false results from antibody interference, particularly in post-COVID samples, how to choose the right blocker and tips to enhance assay performance.
Immunoassay Blocking Agents : Reducing False Results from Antibody Interference Safeguard your immunoassays with proven blocking agents
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PRECISION STARTS WITH CLEAN SIGNALS
Immunoassay blockers are essential tools for reducing false results caused by non- specific binding and antibody interference— such as HAMA and rheumatoid factor (RF). Used in ELISA and lateral flow assays, blockers improve assay accuracy by increasing the signal-to-noise ratio. They are especially important across common assay types—sandwich antigen detection, antibody capture, and competitive ELISAs—where selecting high-affinity antibodies and the right blocking agents is critical to achieving high specificity and sensitivity. Since COVID, many assay developers have reported increased immunoassay interference. One likely contributor is the rise in circulating autoantibodies—especially IgM rheumatoid factor (RF)—following SARS-CoV-2 infection. Studies have shown that COVID-19, like other respiratory viral infections, can trigger lasting immune dysregulation, leading to the emergence of polyreactive antibodies with broad binding capabilities. ⎕ These antibodies, particularly IgM-RF, can bind to the Fc region of assay antibodies—especially those derived from animal sources—forming immune complexes that mimic true signal generation. The result: false positives, elevated background, and reduced assay specificity, ultimately undermining clinical confidence and diagnostic reliability. As this challenge grows, many assay developers are re- evaluating their blocking strategies and exploring new solutions to mitigate interference in increasingly complex, post-COVID sample populations.
Detection Antibody
Detector Label
Capture Antibodies
IgM RF
IMMUNE COMPLEX MIMICKING TRUE ASSAY SIGNAL
⎕ Mohan, A., et al. (2023). Navigating the Post-COVID-19 Immunological Era: Understanding Long COVID-19 and Immune Response. Life 13(11), 2121. https://doi.org/10.3390/life13112121
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CHOOSING THE RIGHT BLOCKER Tailored Blocking Solutions for Every Stage of Development
Accurate immunoassays depend not only on high-quality antibodies, but also on effective blocking strategies that minimize background and interference—especially from HAMA, rheumatoid factor (RF), and other heterophilic antibodies. Whether you are developing ELISAs, lateral flow assays (LFA), or chemiluminescent immunoassays (CLIA), Meridian’s blockers are your first line of defense. Our portfolio includes trusted solutions such as: • Mouse Serum • Mouse IgG • TRU Block TM Series (TRU Block TM , TRU Block TM 2, TRU Block TM 3) • K-BLOCK TM Unsure which blocker fits your assay? Scan to learn more and order a sample pack
Explore Meridian’s complete range of interference-blocking reagents:
BLOCKER
BLOCKER TYPE
INTERFERENCE TYPE
ASSAY COMPATIBILITY
Passive Passive
HAMA, non-specific binding proteins ELISA, LFA
Mouse Serum
HAMA
ELISA, CLIA ELISA, CLIA
Mouse IgG
TRU Block TM
Universal (Active + Passive)
HAAA, RF, HA
ELISA, CLIA, LFA
TRU Block TM 2 TRU Block TM 3
ELISA, CLIA
Active
HAAA, RF, HA
ELISA, CLIA, LFA
K-BLOCK TM
Need help selecting the right blocker? Visit our Blocker Resource Page or contact our technical team for personalized guidance.
Mouse IgG Blocker & Mouse Serum
Neutralizing HAMA for Cleaner Results Mouse IgG blocker is a highly purified immunoglobulin designed to block human anti-mouse antibodies (HAMA), which are a common source of interference in assays using mouse monoclonal antibodies. It binds to HAMA in the sample, preventing it from interacting with assay antibodies. Mouse serum, on the other hand, offers a broader blocking profile. It contains a natural mix of mouse proteins, including immunoglobulins, making it ideal for assays where multiple types of interference may be present.
WHEN TO USE EACH
BENEFITS
• Reduces false positives and false negatives • Improves assay reproducibility • Compatible with ELISA, CLIA, and LFA formats
Mouse IgG Blocker
Mouse Serum
For targeted HAMA interference in high- sensitivity assays
For broader interference blocking in early-stage assay development or when multiple interferences are suspected
ORDERING INFORMATION Mouse Serum
Cat. No.
Cat. No. A66181M A66185M
Mouse IgG
88131
Heat Inactivated
22—28 mg/mL 50—55 mg/mL
N14010M
Normal
A66185M-NA
50—55 mg/mL, Azide Free
A66186M
9—13 mg/mL
A66186M-NA
9—13 mg/mL, Azide Free
A66187M A66189M
10—20 mg/mL
45—55 mg/mL, Proclin 30—40 mg/mL, Azide Free
A66190M-NA
WHEN TO USE BLOCKERS: ASSAY FORMATS AT RISK OF INTERFERENCE
SEROLOGY (IgM, IgG, IgA)
SANDWICH ASSAY
COMPETITIVE ASSAY
Detector Labels
Inhibitor Antigens Patient Analytes Detector Labels
Secondary Antibody
Detector Labels
Detection Antibodies
Detection Antibodies
Patient’s IgG Ab Patient’s IgM Ab
Antigens
Capture Antibodies
Capture Antibodies
Antigens
DIRECT DETECTION
INDIRECT DETECTION
IgG DETECTION
IgM DETECTION
DIRECT DETECTION
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Tiered Blocking for Every Challenge TRU Block TM is Meridian’s flagship line of interference blockers, designed to address a wide range of assay challenges. All TRU Block TM formulations consist of varying combinations of mouse IgG and a proprietary active blocker. Each version is optimized for different levels of interference and assay sensitivity. TRU Block TM Series
UNDERSTANDING THE DIFFERENCES BETWEEN TRU BLOCK TM FORMULATIONS
TRU Block TM
TRU Block TM 2
TRU Block TM 3
• Heavier on the active blockers, lighter on the mouse IgG • Validated on ELISA and CLIA
• A balance between active blockers and mouse IgG • Validated on ELISA, CLIA, and LFA
• Heavier on the mouse IgG, lighter on the active blockers • Validated on ELISA and CLIA
Proprietary Active Blocker Mouse IgG
All three formulations are universal blockers and work against a range of interferences including HA, HAMA, and RF.
WHY CHOOSE THE TRU BLOCK TM SERIES? • Proven performance across platforms • Minimal impact on assay kinetics • Available in bulk and customizable formats
ORDERING INFORMATION HAMA & RF Blockers
Cat. No. A66800H A66802H A66803H
TRU Block TM TRU Block TM 2 TRU Block TM 3
The TRU Block TM Series demonstrates superior HAMA and RF blocking over mouse IgG and other commercial active blockers
( A ) HAMA INTERFERENCE SUPPRESSION
( B ) RF INTERFERENCE SUPPRESSION
Mouse IgG
TRU Block TM
TRU Block TM 2
TRU Block TM 3
Competitor (Active)
Blocker Concentration ( μ g/mL)
Blocker Concentration ( μ g/mL)
Percentage of ( A ) HAMA interference and ( B ) RF interference blocked by Mouse IgG , each of the TRU Block TM Series , and the leading competitor active blocker , at various concentrations of blocker. Higher blocking (%) indicates better blocking performance. The TRU Block TM Series outperforms the leading competitor at lower concentrations in (A), and at all concentrations in (B).
K-BLOCK TM
Broad-Spectrum Blocking Power & Animal-Free
K-BLOCK TM is a proprietary blend of recombinant immunoglobulins and proteins that offers broad-spectrum active blocking against a wide range of interfering substances, including: • Heterophilic antibodies • Rheumatoid factors • HAAAs (e.g., HAMA, HAGA, HARA, etc.) ORDERING INFORMATION K-BLOCK TM BN1200
KEY FEATURES • Effective across multiple species and sample types • Ideal for lateral flow and point-of-care assays • Recombinant, animal-free blocker
APPLICATIONS
Lateral flow devices ELISA and CLIA platforms High-throughput screening
K-BLOCK TM demonstrates superior HAMA and RF blocking over mouse IgG and other commercial active blockers
( A ) HAMA INTERFERENCE SUPPRESSION
( B ) RF INTERFERENCE SUPPRESSION
Mouse IgG
K-BLOCK TM
Competitor (Active)
Blocker Concentration ( μ g/mL)
Blocker Concentration ( μ g/mL)
Percentage of ( A ) HAMA interference and ( B ) RF interference blocked by Mouse IgG , K-BLOCK TM , and the leading competitor active blocker , at various concentrations of blocker. Higher blocking (%) indicates better blocking performance. K-BLOCK TM outperforms the leading competitor at lower concentrations in (A), and at all concentrations in (B).
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SPECIAL CONSIDERATIONS:
IgM Serologic Assays IgM serologic assays commonly encounter two types of interference. The first stems from high levels of patient IgG antibodies, which can compete with IgM for antigen binding sites on the solid phase. Because IgG makes up approximately 75% of total serum antibodies, it can easily outcompete IgM due to its abundance. The second type of interference is caused by rheumatoid factor (RF), which can bind to the Fc region of immunoglobulins causing false-positive results. To address these issues, IgM serologic assays typically require two types of blockers: a HAMA/ RF blocker to reduce RF interference, and an IgG absorbent to prevent interference from the more abundant and potentially cross-reactive IgG antibodies.
RECOMMENDED BLOCKERS FOR IgM ASSAYS
IgG Absorbent
IgM Assay Diluent
L15406G
8120
Goat Anti-Human IgG Fc
ELISA IgM Assay Diluent I
IgG Absorbent is a purified goat anti-human IgG (GAH IgG Fc) Fc fragment designed for the removal of human IgG and IgG/rheumatoid factor (RF) complexes from serum prior to testing for specific IgM antibodies in ELISA or other immunoassays. Removal of IgG interference has been demonstrated to significantly increase the sensitivity of IgM detection in immunoassays. Meridian’s IgG Absorbent is tested by immunoelectrophoresis to ensure specificity for IgG and no cross-reactivity to IgM or IgA.
IgM Assay Diluent is intended for use in qualitative and quantitative assays that detect IgM antibodies. This diluent is formulated to reduce assay interference from a patient’s IgG antibodies, rheumatoid factor, heterophilic antibodies, and other non-specific proteins that can affect the immunoassay results. Blocking proteins within the IgM Assay Diluent are in high excess and will bind to any open binding site on the assay solid phase to prevent binding from interfering factors. When included as a part of a test kit, IgM Assay Diluent can increase the sensitivity of an IgM detection assay. In a separate tube, dilute the patient serum sample in the IgM Assay Diluent at a 1:21 dilution or greater (mix well). The diluent must be standardized with the other assay components.
RECOMMENDED PROTOCOLS
Dilute prior to adding to patient sample. Recommend diluting 1:10 in PBS. Add in a ratio of 1:10 to patient sample and allow to incubate 5-30 minutes.
TIPS TO ENHANCE ASSAY PERFORMANCE Blocker Use Guidelines ELISA
CLIA Add to the reaction mix during the assay setup
Lateral Flow Assay Add to the conjugate pad, sample buffer, or pre-treatment buffer, or apply directly to the membrane as a blocking stripe ahead of the test stripe
Add to the sample or conjugate diluent, but ideally should be in contact with the patient sample before incubation with the assay capture antibody
PRODUCT
RECOMMENDED CONCENTRATION
Mouse Serum
5-20% of the sample or conjugate diluent (v/v)
Mouse IgG
0.1-2 mg/mL or 10x the concentration of the conjugate MAb used in the assay
TRU Block TM Series Diluted samples: 0.5-20 μ g/mL, undiluted samples: 5-200 μ g/mL K-BLOCK TM 0.2-200 μ g/mL IgG Absorbent Dilute prior to adding patient sample (1:10 in PBS) in a ratio of 1:10 IgM Assay Diluent Warm diluent to room temp. before use and add to patient sample at a 1:21 ratio or greater
FREQUENTLY ASKED QUESTIONS
What is the difference between passive blockers and active blockers? Passive blockers (e.g., mouse IgG, goat IgG) bind to non-specific sites or interfering antibodies in the sample to reduce background noise. Active blockers , such as K-BLOCK TM , contain targeted formulations designed to neutralize specific interfering antibodies like HAMA or RF, offering a higher degree of protection in complex sample matrices. When should I use TRU Block TM instead of Mouse IgG? Mouse IgG is a passive blocker that helps reduce HAMA interference by competitive binding but may require high concentrations that can affect assay performance. TRU Block TM is an active blocker designed to more effectively neutralize a broader range of interfering antibodies, including HAMA and RF, even at lower concentrations. Use Mouse IgG for general interference protection; choose TRU Block TM for high-risk samples or when stronger, more targeted blocking is needed—such as in post-COVID or autoimmune populations. What types of assay benefit most from using blockers? Blockers are commonly used in ELISA , lateral flow assays (LFAs) , and chemiluminescent immunoassays (CLIA) — particularly in sandwich and antibody capture formats where interference is more likely to generate false positives. How do I determine the best blocker for my assay? It depends on:
Species of your assay antibodies
Assay format (e.g., sandwich vs. competitive)
Sample type (e.g., serum, plasma, whole blood)
Known sources of interference (e.g., RF, HAMA)
Contact Meridian’s technical team for blocker recommendations tailored to your assay type. What is the most common immunoassay interference? Human anti-mouse antibodies (HAMA) are the common antibody interference which causes both positive and negative interferences in two-site mouse monoclonal antibody-based assays. Is HAMA the only type of heterophilic antibody interference? No. HAMA only represents one type of heterophilic antibody (HA) interference—others include HA to animals such as goat (HAGA) , sheep (HASA) , and rabbit (HARA) which can cause false results when antibodies originating from these animals are used in immunoassays. In addition to HA, there is another class of interference called rheumatoid factor (RF) which is an autoantibody that reacts with the patient’s own immunoglobulin (Ig) and can cross react with animal Ig, similar to HA/HAMA interference. What is rheumatoid factor (RF)? Rheumatoid factor is an autoantibody directed against the Fc portion of IgG. The most common type and the main concern in immunoassay interference is IgM RF because it’s a pentamer —this makes it highly effective at crosslinking and forming immune complexes. IgM RF is common in people with rheumatoid arthritis, occurring in about half of all people with the disease—but about 1 in 20 people without rheumatoid arthritis can also have high levels and it is more common in elderly patients. How does rheumatoid factor (RF) interfere with immunoassays? RF, similar to heterophilic antibodies, has the potential to cause falsely elevated test results by crosslinking the assay antibodies, even in the absence of analyte—most often via binding to the Fc portion of assay antibodies. RF can react against different species of IgG, including human and rabbit. How can you protect your immunoassay from post-COVID immune changes? COVID-19 has led to a rise in circulating autoantibodies—particularly polyreactive forms of IgM rheumatoid factor (RF)— which can interfere with immunoassay performance. To protect assay accuracy, many developers are now validating with post-COVID clinical samples and incorporating high-performance blockers to mitigate the risk of false positives.
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