How Lateral Flow Immunoassays Work: Sandwich Format Explained

Multi-biomarker test strips can simultaneously measure six biomarkers using a single sample. To prevent interference between markers, flow direction is designed from bottom to top rather than left to right. Each marker line produces a test peak relative to a control peak, with the ratio indicating biomarker concentration. This multiplexed approach improves diagnostic discrimination between infected contacts and those who develop active disease.

Multiplex lateral flow strips can simultaneously detect general pathogen positivity and identify specific serotypes. Two types of sandwich immunoassays are used: homologous sandwiches (same antibody for capture and detection) for general detection, and heterologous sandwiches (different antibodies) for serotype-specific identification. Probe concentration optimization (0.5-2 OD) and test line positioning affect signal intensity, with closer lines showing higher signals due to increased interaction time between detection antibody and antigen.

Multiplex testing allows detection of multiple pathogens (such as flu A, flu B, and SARS-CoV-2) in a single test. This technology was accelerated through federal programs like RADx. Single-pathogen tests are becoming obsolete because respiratory infections often involve multiple potential causes, and appropriate treatment depends on accurate diagnosis. Future tests will likely detect four or more pathogens simultaneously.

The RFP seeks multi-pathogen multiplex platforms but acknowledges that multiplexing may be part of an applicant's R&D plan if they currently singleplex. The optimal design specification requires detecting four or more pathogens simultaneously from a single sample, while the acceptable specification allows detection of one pathogen. Scoring guidelines award points based on the number of pathogens detectable in a single reaction. Multiplex capacity is weighted heavily (denoted by 3) and must be specifically addressed in applicant presentations and RD plans. Applicants with extensive test menus but no current multiplexing capability can still score points by describing their plans to multiplex key pathogens.

Multiplexing in lateral flow assays allows the simultaneous detection of multiple target analytes in a single test. This is achieved through three main formats: (1) Multi-line design where several test lines are printed behind each other with different capture molecules, (2) Parallel channel design with separate channels for each target molecule, and (3) Combined approaches using both multi-line and parallel channel designs. These formats enable applications like drug testing and disease diagnosis by detecting multiple biomarkers simultaneously.

Prozone phenomenon (also called hook effect or antibody excess phenomenon) is an immunological interference where high antibody concentrations prevent proper immune complex formation, causing false-negative results. In the zone of equivalence, adequate antibody-antigen ratios produce visible flocculation. In prozone, excess antibodies coat antigens incompletely, preventing precipitation. Post-zone phenomenon occurs with antigen excess. Both cause false negatives. This is particularly common in HIV/syphilis co-infection due to excessive B-cell stimulation, pregnancy, early syphilis stages, and neurosyphilis. Laboratories should dilute patient serum to reach the zone of equivalence for accurate results.

Antigen detection rapid diagnostic tests (RDTs) work through immunochromatography using a nitrocellulose membrane strip where a conjugate of anti-antigen antibody coupled to colloidal gold migrates with patient samples; if the antigen is present, it binds to the conjugate and gets captured by test line antibodies, producing a visible purple-red line, while the control line always appears purple regardless of test results. However, two critical artifacts can affect accuracy: the prozone effect occurs when high antigen concentrations cause free antigens to outcompete conjugate binding, leading to false negatives; the backflow effect happens when tests are read too late and dried strips allow reverse migration, potentially causing false positives.

This section covers antibody-related interferences and the hook effect. Heterophile antibodies can interfere with multiple assays including thyroid function tests, insulin assays, and rheumatoid factor assays, causing both positive and negative interference. Detection methods include precipitation, different analytical platforms, heterophile blocking anti-tubes, and serial dilutions. The hook effect affects single-step sandwich immunoassays where excess antigen causes falsely low results in prolactin, HCG, growth hormone, and tumor marker assays. A variant hook effect can occur with variant HCG forms in pregnancy, causing falsely negative pregnancy test results.

Five major sources cause immunoassay interference: (1) Human endogenous antibodies (heterophilic, anti-animal, autoantibodies, therapeutic antibodies) causing positive/negative interference; (2) Cross-reactivity with closely related proteins, varying with temperature; (3) Prozone/hook effect where high analyte concentrations reduce immune complex formation; (4) Elevated endogenous serum components (bilirubin, hemoglobin, lipids, proteins) interfering more in homogeneous assays; (5) Matrix effects from sample components interfering with antibody-protein binding. Resolution strategies include sample dilution (effective for prozone, matrix, and elevated components), recognizing interference patterns, and implementing automated analyzer flags for suspicious results.

Lateral flow immunoassays, exemplified by pregnancy tests, represent the most common paper-based diagnostic technology. These binary tests work well for conditions with clear yes/no outcomes but are limited for quantitative analysis. Wax printer technology enables mass production of paper-based diagnostic devices by printing wax patterns onto paper, which absorb when heated. These printers cost approximately $800 and can produce about 10 million tests per year when operating 24 hours daily. The printing process takes about two seconds per device. Smartphone camera phones can capture images of paper-based diagnostic devices, which are then sent to central laboratories for analysis. This approach eliminates the need for doctors to be present at the point of care, allowing healthcare workers to collect samples and transmit data while computers perform complex analysis.

Lateral flow tests offer significant advantages in portability, accessibility, and cost but face inherent trade-offs in sensitivity compared to molecular PCR testing. Negative results do not definitively rule out disease—they simply indicate viral loads below detection limits. When results reach or exceed detection thresholds, sensitivity and specificity achieve very high levels appropriate for their intended clinical applications. Future developments will include novel target diseases, integrated DNA/RNA amplification to enhance sensitivity, expanded reporter particle options beyond gold nanoparticles, and introduction of alternative capture mechanisms. Most significantly, digital readers utilizing smartphone image processing represent a transformative advancement, enabling automatic result interpretation, eliminating visual interpretation uncertainty, and supporting lay person use while maintaining clinical traceability and data integrity.

Rapid diagnostic tests use lateral flow strips where samples migrate via capillary action through membranes containing capture antibodies and control lines. When target biomolecules bind to detection antibodies conjugated to nanoparticles, visible signals appear at test lines. This technology provides qualitative yes/no results similar to pregnancy tests but can be adapted for quantitative measurements. The integration of luminescent nanoparticle probes with smartphone readers enables portable, accessible diagnostic capabilities with laboratory-grade sensitivity.

Lateral flow assays can be used for both serology (antibody) testing and antigen testing, as well as detecting chemical and biological markers. The technology is scalable globally with manufacturers in Taiwan, India, Brazil, and the US. A universal lateral flow assay reader standardizes tests, improves sensitivity, eliminates human error, and collects metadata including vaccination status and disease history. The reading of each assay takes approximately 1-2 seconds, with machines capable of processing 500 samples per hour. Quantitative lateral flow assays can be developed by calibrating against PCR or ELISA results. Nucleocapsid antibodies are the best candidate for serology testing because they are completely discriminative between natural infection and vaccination. The nucleocapsid protein is more stable and does not mutate like spike, making it a reliable marker for natural immunity. Finger prick tests are not sensitive enough; venous blood draw is required.

Quantitative lateral flow assays require comprehensive understanding of device architecture, test formats, and analytical principles. The device comprises four components: sample pad, conjugate release pad, nitrocellulose membrane with test/control lines, and absorption pad. Two primary test formats exist: sandwich tests for macromolecular analytes with multiple binding sites (producing increased signal intensity) and competitive tests for small molecules with single binding sites (producing decreased signal intensity). Quantitative tests differ from qualitative tests by determining exact analyte concentrations rather than mere presence/absence. Key parameters include Limit of Detection (LOD), Lower Limit of Quantification (LLOQ), and Upper Limit of Quantification. System uncertainty, representing standard variation around mean values, must be minimized. Sample variability significantly impacts performance, as demonstrated by human urine pH fluctuating between acidic and alkaline states. Material selection critically affects performance through capillary flow time: slow membranes cause false positives while fast membranes cause false negatives. Reagents including antibodies and hapten-carrier conjugates demand strict quality control with batch-specific standard curves.