Residual Detection Kits: Principles, Applications, and Future Prospects

What are residual detection kits, and why are they so important in modern industries?

Residual detection kits are pre-made, integrated detection tools developed based on immunological, molecular biological, or biochemical principles. Their core function is to rapidly, specifically, and sensitively qualitatively or quantitatively detect extremely low concentrations of specific residual substances in samples. These residues typically include impurities in biopharmaceutical production processes (such as host cell proteins HCP, host cell DNA, Protein A, etc.), harmful substances in food (such as pesticide residues, veterinary drug residues, mycotoxins, allergens), environmental pollutants (such as heavy metals, antibiotics), and cleaning agent residues on medical devices, among others. In the process of modern industrialization, the importance of residual detection kits has reached an unprecedented strategic level. Their value is first reflected in safeguarding public health and safety, such as ensuring that pesticide residues in food do not exceed standards and that harmful production impurities are absent in pharmaceuticals, directly impacting consumer life and health. Secondly, they are core tools for ensuring product quality and compliance. Whether for pharmaceuticals, biologics, or food, production must strictly adhere to stringent residual limit standards set by regulatory agencies worldwide (such as the U.S. FDA, EU EMA, and China NMPA). These kits provide reliable data support and are indispensable in product release and market applications (such as BLA and NDA). Furthermore, they significantly enhance the efficiency and economic benefits of production process control. Through rapid online or at-line testing, production processes (such as chromatography column cleaning effectiveness and virus inactivation efficiency) can be monitored in real time, deviations detected promptly, and entire batch product discards avoided, thereby saving substantial costs. Thus, although residual detection kits are small, they serve as the "gatekeepers" and "referees" supporting high-quality development in fields such as food safety, pharmaceutical manufacturing, biotechnology, and environmental monitoring.

What core technical principles do residual detection kits primarily rely on, and how do they work?

Residual detection kits employ diverse technical principles, but their design purpose is to achieve highly specific and sensitive capture and signal conversion of target residues. Current mainstream technology platforms include enzyme-linked immunosorbent assay (ELISA), lateral flow immunoassay (LFIA)-based rapid test strips, fluorescent immunoassay (FIA), chemiluminescent immunoassay (CLIA), and emerging technologies such as biosensors and qPCR/dPCR.

Enzyme-linked immunosorbent assay (ELISA) is currently the most widely used and mature platform, particularly suitable for the quantitative detection of proteins (e.g., HCP) and small molecule haptens (e.g., pesticides, toxins). Its working principle is based on the specific reaction between antigens and antibodies and the enzyme-catalyzed color development of substrates. Taking sandwich ELISA (commonly used for macromolecule detection) as an example: the inner walls of the microplate in the kit are pre-coated with capture antibodies. During detection, the sample is added, and the target antigen (residue) in the sample is specifically bound and fixed to the plate by the capture antibodies. After washing away unbound substances, another detection antibody conjugated with an enzyme (e.g., HRP horseradish peroxidase) is added. This antibody binds to another epitope of the target antigen, forming an "antibody-antigen-enzyme-labeled antibody" complex. After washing again, a color-developing substrate (e.g., TMB) for the enzyme is added. The enzyme catalyzes a chemical reaction in the substrate, producing color. The depth of the color is proportional to the concentration of the target antigen in the sample. Finally, the absorbance is measured using a microplate reader, and the residual content is accurately calculated by comparing it to a standard curve. The advantages of ELISA include high sensitivity (reaching pg/mL levels), strong specificity, high throughput, quantifiability, and relatively controllable costs, making it the gold standard for laboratory quantitative detection.

Lateral flow immunoassay (LFIA) test strips are renowned for their "rapid, simple, and low-cost" characteristics, making them highly suitable for on-site screening and use in grassroots units. Their structure typically consists of a sample pad, conjugate pad, nitrocellulose membrane, and absorbent pad. During detection, the sample solution is dropped onto the sample pad and flows forward via capillary action. When it reaches the conjugate pad, the target in the sample binds to the labeled antibodies (typically labeled with colloidal gold or colored microspheres) pre-lyophilized there. The complex then continues to flow to the test line (T line), where another antibody against the target (or an antigen-protein conjugate for small molecules) is fixed, capturing the complex and causing color aggregation. The control line (C line) indicates whether the detection process is valid. Results (one or two colored bands) can typically be observed with the naked eye within 10–15 minutes. Although LFIA is generally used for qualitative or semi-quantitative detection, its convenience and speed are irreplaceable.

For nucleic acid residues, such as host cell DNA detection, quantitative PCR (qPCR) technology is the absolute mainstream. The kit includes primers and probes designed for highly conserved sequences of specific species (e.g., Alu sequences for human cells, COS-TR sequences for CHO cells), PCR reaction solution, and DNA standards. By monitoring the accumulation of fluorescent signals during the PCR amplification process, trace amounts of DNA residues in the sample can be quantified with extremely high sensitivity (fg level). Additionally, chemiluminescent immunoassay (CLIA) is increasingly used in high-end detection due to its wider linear range and higher sensitivity compared to ELISA. Biosensor technology represents the future direction, combining biological recognition elements (antibodies, nucleic acid aptamers) with physical sensors (optical, electrochemical) to enable real-time, continuous online monitoring.

In which key areas do residual detection kits have irreplaceable applications?

Residual detection kits are widely used in almost all modern industries with stringent requirements for purity and safety.

In the biopharmaceutical field, their application is mandatory and critical. During the production of biologics such as monoclonal antibodies, recombinant proteins, vaccines, and cell and gene therapy products, various process-related impurities are inevitably introduced. Host cell protein (HCP) residue is one of the primary risks, as even trace amounts of HCP may cause unpredictable immune reactions in humans. Therefore, using high-sensitivity ELISA kits to monitor HCP residues in purified products is a core quality control point in process development and product release. Similarly, host cell DNA residue is theoretically potentially tumorigenic, and pharmacopoeias worldwide strictly limit its residue levels (typically ≤10 ng/dose), necessitating strict quantification using qPCR kits. Additionally, Protein A affinity chromatography media used for protein purification may shed, and shed Protein A is immunotoxic, requiring detection with specialized ELISA kits.

In the food safety field, residual detection kits are the first line of defense in safeguarding "safety on the tip of the tongue." Pesticide residue detection kits (often in ELISA or LFIA formats) are widely used for large-scale screening of vegetables, fruits, and grains, rapidly detecting common pesticides such as organophosphates and pyrethroids. Veterinary drug residue detection kits are used to monitor antibiotics (e.g., chloramphenicol, sulfonamides) and prohibited drugs such as clenbuterol in livestock and poultry products (meat, eggs, milk), ensuring the safety of animal-derived foods. Mycotoxin detection kits (for aflatoxins, deoxynivalenol, ochratoxin A, etc.) are crucial for controlling contamination in grains, nuts, and their products, as these toxins are highly carcinogenic. Food allergen detection kits (for peanuts, gluten, milk, soy, etc.) are vital for protecting the lives of allergic individuals and are commonly used for cleaning validation and finished product testing on food production lines.

In environmental monitoring, medical device cleaning validation, and life science research, residual detection kits also play key roles. Environmental monitoring personnel use them to rapidly screen for pollutants such as heavy metals, herbicides, and hormones in water and soil. Hospitals and medical device manufacturers use specific kits to verify whether cleaning agents (e.g., SDS), disinfectants, or product residues on surgical instruments, bioreactors, and chromatography systems have been completely removed to prevent cross-contamination. In research, they are reliable tools for assessing the purity of experimental processes such as cell culture and protein purification.

What challenges do current residual detection kits face, and how will future technologies evolve?

Although residual detection kit technology is already highly mature, it still faces multiple challenges. The primary challenge is complex matrix interference. The samples to be tested (e.g., cell culture supernatants, food extracts, soil samples) have extremely complex compositions, and the large amounts of non-target proteins, lipids, salt ions, and other substances they contain may cause non-specific binding or signal suppression/enhancement, leading to false-positive or false-negative results. This places extremely high demands on the development of kits with strong anti-interference capabilities. Secondly, the demand for ultra-high throughput and ultra-rapid detection is increasingly urgent. Traditional ELISA detection typically takes several hours, which cannot fully meet the needs of real-time process monitoring (PAT) in modern production lines. Thirdly, there is a lack of multiplex detection capabilities. In practical scenarios, it is often necessary to detect multiple residues simultaneously (e.g., multiple pesticides, multiple allergens), but current single-target detection modes are inefficient and costly. Fourthly, the development of detection methods for new types of residues lags. With the emergence of new chemical materials, new pesticides, and novel biological therapeutic products, corresponding residual risks also arise, requiring detection technologies to respond rapidly.

To address these challenges, residual detection kit technology is rapidly evolving in the following directions:

Integration of new technology platforms: Faster chemiluminescence and electrochemiluminescence (ECL) technologies are gradually replacing traditional colorimetric ELISA to achieve higher sensitivity and shorter detection times. Digital PCR (dPCR) provides absolute quantification and excellence performance independent of standard curves for DNA residue detection.

Popularization of multiplex detection technologies: Multiplex suspension arrays based on Luminex xMAP technology allow simultaneous detection of up to 50–100 targets in a single reaction well, greatly improving detection efficiency, particularly for HCP profiling, multiple allergen, or multiple toxin screening.

Innovation in on-site rapid quantitative technologies: New-generation portable fluorescent immunoanalyzers combined with fluorescent microsphere-labeled LFIA test strips are transforming on-site rapid detection from qualitative to precise quantitative, with traceable data, significantly enhancing the reliability of on-site testing.

Development of new recognition elements: In addition to traditional antibodies, nucleic acid aptamers are being developed for constructing new aptasensors due to their advantages of easy synthesis, high stability, and designability for small molecules, providing new options for detection kits.

Automation and intelligent integration: Integrating kits with fully automated analyzers to achieve full-process automation and intelligence from sample addition, incubation, washing, and reading to result analysis and transmission is an inevitable trend for ultimately eliminating human error and improving laboratory throughput and data consistency.

Conclusion: What is the future outlook for residual detection kits?

As a crystallization of interdisciplinary efforts, the development level of residual detection kits directly reflects our ability to ensure quality and safety at the microscopic scale. From initial qualitative test strips to today's high-throughput, automated, and intelligent integrated systems, they have grown into an indispensable strategic emerging industry. In the future, with the deep integration of new materials, new biotechnologies, microfluidic technologies, and artificial intelligence, residual detection will become more sensitive, rapid, simple, low-cost, and multi-target. The ideal scenario is: at key control points of production lines, an online monitoring probe integrated with microfluidics and biosensors is uploading residual data to a central control system in real time; at customs and supermarkets, staff can accurately screen dozens of risk substances in goods within minutes using handheld devices; in laboratories, fully automated platforms process samples around the clock, analyzing and predicting vast amounts of detection data through AI algorithms. The evolution of residual detection kits will undoubtedly provide solid technical support for building a safer, healthier, and more efficient world.

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