Why Do Clinical Laboratories Mainly Use Immunoassays for Macromolecule Testing, While Increasingly Using Mass Spectrometry for Small-Molecule Testing?

Modern clinical testing relies primarily on two major categories of analytical platforms: immunoassay and liquid chromatography-tandem mass spectrometry (LC-MS/MS). The former is based on antigen-antibody recognition and has long been used for a large number of routine tests, including proteins, hormones, tumor markers, and inflammatory markers; the latter centers on chromatographic separation and mass spectrometric structural identification and is rapidly expanding in small-molecule assays such as vitamins, steroid hormones, therapeutic drug monitoring, and metabolite analysis.

This division of labor is not simply determined by how advanced the instruments are. Although clinical mass spectrometry technology has developed rapidly in recent years, most protein-based markers in hospital laboratories are still measured by immunoassay, while an increasing number of small-molecule assays are shifting toward LC-MS/MS. The fundamental reason lies in the significant differences among the analytes themselves. Macromolecules usually have multiple epitopes that can be recognized by antibodies, making them suitable for building high-throughput, automated immunoassay systems; small molecules, by contrast, are small in size, have limited epitopes, and often have many structural analogs, so the limitations of antibody recognition are more likely to translate into quantitative errors.

Why Macromolecule Testing Is Well Suited to Immunoassay

Proteins, peptides, and some macromolecular biomarkers have relatively complex three-dimensional structures. A protein molecule typically contains multiple epitopes, and different regions can be recognized simultaneously by different antibodies. This feature makes the double-antibody sandwich method a highly effective technical format for macromolecule testing. In a typical sandwich immunoassay, one capture antibody is immobilized or participates in capturing the target molecule, while another detection antibody binds to another epitope on the target molecule. The system generates an effective detection signal only when both antibodies recognize the same target analyte at the same time.

This dual-recognition mechanism significantly improves assay specificity. A single antibody may have some cross-reactivity with structurally similar molecules, but the probability that two antibodies will simultaneously misrecognize the same interfering substance is markedly reduced. Therefore, for common clinical assays such as AFP, CEA, TSH, BNP, CRP, and insulin, the sandwich method can achieve good resistance to interference and detection stability in complex serum or plasma matrices. For hospital laboratories, this means the method can be standardized and automated and is suitable for continuous operation in high-sample-volume settings.

Another characteristic of macromolecules is that the pathways for signal amplification are relatively mature. After the target protein is captured by antibodies, highly sensitive detection can be achieved through enzyme labels, chemiluminescence, electrochemiluminescence, or fluorescence systems. Because the target itself usually has a sufficiently large structural interface, antibody binding can both provide selectivity and establish a stable carrier for the detection signal. This is also an important technical foundation for the long-standing dominance of immunoassays in protein-based clinical markers.

Why Immunoassay Has Become the Mainstream Platform in Hospitals

From the perspective of laboratory operations, one of the greatest advantages of immunoassay is its high degree of automation. Modern fully automated immunoassay analyzers can perform steps such as sample identification, pipetting, incubation, washing, signal detection, result calculation, and quality control management. For hospital clinical laboratories, this automation capability directly corresponds to high throughput, low dependence on manual labor, and stable turnaround times.

Hospitals need to process large numbers of routine testing samples every day, and testing platforms must not only be accurate but also suitable for continuous operation. Immunoassay analyzers are usually connected to laboratory information systems and can support barcode management, result uploading, and abnormal-result alerts. For high-frequency assays such as AFP, CEA, TSH, BNP, and CRP, laboratories are more concerned with whether the method is stable, whether throughput is sufficient, whether maintenance is simple, and whether reagent lots are consistent. Immunoassay has already developed a mature industrial chain in these areas, so its status as the mainstream hospital platform has an obvious practical basis. This does not mean that immunoassay is superior to mass spectrometry in all scenarios. Its advantages are mainly concentrated in assays that are suitable for antibody recognition and can be automated. When the analytes shift from macromolecules to small molecules, the inherent limitations of antibody recognition become more pronounced.

The Inherent Limitations Faced by Small-Molecule Testing

Small molecules such as drugs, hormones, vitamins, and metabolites are usually small in size, with limited structures on the molecular surface available for antibody recognition. Many small molecules cannot even accommodate the binding of two antibodies at the same time, making it difficult to establish a double-antibody sandwich method. For these types of analytes, immunoassays typically use a competitive format.

The basic principle of competitive immunoassay is to have the target small molecule in the sample and a labeled small-molecule analog compete together for a limited number of antibody binding sites. The higher the concentration of the target analyte in the sample, the less labeled compound can bind to the antibody, and the lower the detection signal usually is. A standard curve is then used to convert the change in signal into the sample concentration. The competitive method solves the problem that small molecules cannot be recognized in a sandwich format by two antibodies, but it also introduces new limitations: the entire recognition process usually depends on a single antibody, and antibody cross-recognition of structural analogs has a more direct impact on the result.

Small-molecule testing also faces another problem: clinical samples often contain large numbers of structurally similar endogenous compounds, metabolites, and drug degradation products. These may differ from the target analyte by only a hydroxyl group, a methyl group, the position of a double bond, or a side-chain structure. For antibodies, these subtle differences cannot always be fully distinguished, so competitive small-molecule immunoassays are inherently more susceptible to cross-reactivity.

Why Antibody Cross-Reactivity Has a Greater Impact on Small Molecules

Antibodies recognize spatial structures and surface chemical features, not chemical names. Even if a kit is labeled as measuring a specific small molecule, what the antibody actually recognizes is the local shape, charge distribution, hydrophobicity, and hydrogen-bonding capability presented by that molecule. Any antibody may have some degree of cross-reactivity; only the extent differs across assays and antibodies. In sandwich-format macromolecule testing, two antibodies jointly participate in recognition. If an interfering substance binds weakly to only one of the antibodies, this is usually insufficient to form a complete signal. Even if a structurally similar protein is present, it would need to have two recognizable epitopes at the same time and be captured by the two antibodies in an appropriate spatial relationship to cause obvious interference. This dual threshold reduces the probability that the sandwich method will misrecognize non-target substances.

Competitive small-molecule testing is different. Because usually only one antibody is responsible for recognition, any structural analog that can bind to that antibody has the opportunity to participate in the competition and thereby change the signal intensity. Vitamin D testing is a typical example. 25-hydroxyvitamin D2, 25-hydroxyvitamin D3, other hydroxylated metabolites, and the efficiency of release from binding proteins may all affect immunoassay results. Similar problems also exist in steroid hormone testing. Testosterone, cortisol, cortisone, aldosterone, and their metabolites have highly similar structures, and antibody cross-reactivity may lead to inconsistent results across different platforms.

Therapeutic drug monitoring and immunosuppressant testing are likewise affected by this problem. Drugs form metabolites in the body, and some metabolites may still retain structural fragments similar to those of the parent drug. If the antibody cannot effectively distinguish the parent drug from its metabolites, the test result may overestimate the true concentration of active drug. For drugs that require dose adjustment and have a narrow therapeutic window, this type of bias can affect clinical interpretation and medication decisions.

Why Mass Spectrometry Is Better Suited to Small-Molecule Analysis

LC-MS/MS does not rely on antibody recognition; instead, it uses the structural characteristics of the molecule itself for confirmation. A small molecule has a specific retention time in liquid chromatography, forms a specific precursor ion after entering the mass spectrometer, and produces characteristic product ions during collision-induced dissociation. Quantitative methods also commonly monitor ion abundance ratios to confirm whether the target analyte conforms to the expected fragmentation pattern. This recognition process is actually completed jointly across multiple dimensions. Chromatographic retention time provides separation information, precursor ion mass reflects molecular mass, product ion mass reflects structural fragments, and the ion abundance ratio further confirms whether the fragmentation behavior is consistent. For structurally similar small molecules, a single dimension may be insufficient for complete differentiation, but after multiple dimensions are combined, mass spectrometry can usually achieve specificity higher than that of competitive immunoassay.

Stable isotope internal standards further improve the reliability of LC-MS/MS quantitation. An ideal internal standard is highly similar in structure to the target analyte and can undergo sample pretreatment, chromatographic separation, ionization, and detection together with it, thereby correcting for matrix effects, sample loss, and fluctuations in instrument response. For assays such as vitamin D, steroid hormones, immunosuppressants, antibiotics, antiepileptic drugs, and various therapeutic drug monitoring tests, stable isotope internal standards are an important component of high-quality mass spectrometry methods.

Why Mass Spectrometry Did Not Become the Mainstream Hospital Platform for a Long Time

Although LC-MS/MS has clear specificity advantages in small-molecule analysis, for a long time it did not become the routine primary platform in all hospitals in the way immunoassay did. The main reason is not that mass spectrometry cannot perform the testing, but that the complete workflow was historically relatively complex. Many clinical mass spectrometry methods require sample pretreatment steps such as protein precipitation, liquid-liquid extraction, solid-phase extraction, derivatization, centrifugation, nitrogen evaporation, and reconstitution. Each step can affect recovery, matrix effects, and between-batch consistency.

For clinical laboratories, method complexity directly affects personnel training, quality control, result turnaround time, and operating costs. Immunoassay analyzers can be highly automated from the original sample through result output, whereas traditional LC-MS/MS often requires experienced technologists to perform sample processing, method maintenance, and data review. This operational barrier has limited the speed at which mass spectrometry has been adopted in small and medium-sized hospitals and in high-throughput routine testing.

In addition, development of mass spectrometry methods themselves requires substantial professional expertise. Laboratories need to select appropriate chromatographic conditions, ion source parameters, MRM transitions, internal standards, calibrators, and quality control materials, and also need to evaluate matrix effects, carryover, linear range, and methodological performance. For the large number of macromolecule assays already covered by mature immunoassay platforms, hospitals usually lack a strong incentive to migrate them to mass spectrometry platforms.

Automated Sample Preparation Is Changing the Industry Landscape

In recent years, automated sample preparation has been changing the application boundaries of clinical mass spectrometry. One of the core bottlenecks that previously constrained the development of clinical mass spectrometry was sample preparation, and this bottleneck is gradually being addressed. Magnetic bead enrichment technology, automated sample pretreatment platforms, automated solid-phase extraction, and robotic sample handling systems are transforming steps that originally depended on manual experience into more standardized workflows.

The value of automated pretreatment is not merely labor savings. More importantly, it can reduce pipetting errors, decrease batch-to-batch variation, improve consistency in sample processing, and make LC-MS/MS easier to incorporate into the routine operating system of hospital laboratories. For example, magnetic bead enrichment can be used for the selective processing of specific analytes or protein conjugates, automated solid-phase extraction can improve cleanup efficiency and reduce matrix interference, and robotic platforms can complete sample addition, mixing, incubation, transfer, and pre-instrument preparation under fixed programs.

As the degree of automation in sample preparation increases, the operational gap between LC-MS/MS and traditional immunoassays is narrowing. Mass spectrometry still requires specialized method maintenance, but its workflow is gradually shifting from an “expert-oriented technology platform” toward a “standardizable clinical platform.” This change will drive more small-molecule assays to move from immunoassays to mass spectrometry, especially in settings where result accuracy, specificity, and multi-analyte testing capability are more highly valued.

Future Development Trends in Clinical Mass Spectrometry

The future development of clinical laboratories is not a simple substitution relationship between immunoassays and mass spectrometry, but rather a clearer technical division of labor based on the characteristics of the analytes. Macromolecular testing will still extensively use immunoassays, because proteins and peptides have multi-epitope structures, and the double-antibody sandwich method has mature advantages in automation, high throughput, and resistance to interference. For assays such as AFP, CEA, TSH, BNP, CRP, and insulin, immunoassays will remain an important foundational platform in hospital laboratories.

Small-molecule testing, however, will increasingly adopt LC-MS/MS. The structural similarity of drugs, hormones, vitamins, and metabolites makes competitive immunoassays susceptible to cross-reactivity, whereas mass spectrometry can confirm target compounds across multiple dimensions, such as retention time, precursor ion, fragment ions, and ion abundance ratios. With the development of automated sample pretreatment, stable isotope internal standards, and standardized method packages, the adoption of clinical mass spectrometry in small-molecule analysis will continue to accelerate.

Summary

The choice of testing platform in clinical laboratories essentially depends on the structural characteristics of the analyte and the operational needs of the laboratory. Macromolecules have multiple epitopes and are suitable for the double-antibody sandwich method, so immunoassays can provide reliable results while ensuring throughput and automation. Small molecules, by contrast, have limited epitopes and many structural analogs, and usually can only be analyzed by competitive immunoassays; therefore, they are more susceptible to interference from antibody cross-reactivity, metabolites, and degradation products.

The advantage of LC-MS/MS lies in the fact that it does not rely on antibodies, but instead uses the structural information of the molecule itself for identification. For small molecules, this structure-based confirmation approach is usually more suitable for addressing specificity issues. As automation in sample preparation gradually matures, the application of mass spectrometry in clinical small-molecule testing will further expand, while immunoassays will also continue to maintain an important position in high-throughput macromolecular testing. The two types of technologies do not replace each other; rather, they each play the role for which they are best suited among different types of analytes.

Related Services from Pulijian Biotechnology

Pulijian Biotechnology (Changzhou) Co., Ltd. focuses on stable isotope internal standards, reference standards related to LC-MS/MS method development, and custom synthesis services, and can provide internal standard selection recommendations and custom support for small-molecule testing projects such as vitamins, steroid hormones, therapeutic drug monitoring, and metabolite analysis. For projects that are transitioning from immunoassays to mass spectrometry, suitable stable isotope internal standards can help improve method robustness, correct matrix effects, and improve quantitative consistency. If you need to select suitable stable isotope internal standards for an LC-MS/MS project, we can provide professional recommendations based on the analyte structure and testing platform, and assist in evaluating isotope labeling positions, mass differences, isotopic purity, and method compatibility.