Introduction
Lateral Flow Assay (LFA), also known as immunochromatographic test strip, is the most successful and widely used technology platform in the point-of-care testing (POCT) field. It offers unique advantages including simple operation, rapid detection, no instrument requirement, low cost, and suitability for single-use testing. It is widely used in infectious disease rapid screening (COVID-19, influenza, HIV), pregnancy testing, drug abuse screening, food safety, veterinary drug residue testing, and other applications. From home self-test COVID-19 antigen test strips to point-of-care testing in clinical departments, LFA has brought immunoassay from central laboratories to primary care facilities and homes. The performance of high-quality LFA test strips heavily depends on the proper selection and combination of antibody pairs, colloidal gold/fluorescent microspheres, nitrocellulose (NC) membranes, sample pads, conjugate pads, absorbent pads, blocking agents, and other raw materials. This article systematically introduces the technical principles, main application areas, key raw material selection points, formulation examples, and frequently asked questions for LFA, providing a complete raw material technical reference for LFA test strip development.
I. Overview of LFA Technology
1.1 Basic Principles of LFA
LFA is based on the principle of capillary chromatography, with immunoassays immobilized on the test strip. After sample application, it migrates along the test strip by capillary action, passing through the conjugate pad, NC membrane (test line and control line), and absorbent pad, completing the immunoassay and detection within minutes.
Double Antibody Sandwich Method Principle (using colloidal gold as example):
1.2 Common LFA Formats
| Format | Principle | Application Scenario | Example |
|---|---|---|---|
| Double Antibody Sandwich Method | Gold-labeled antibody and capture antibody recognize different epitopes of antigen | Large molecule antigen detection (viruses, proteins) | COVID-19 antigen, HCG |
| Competitive Method | Sample antigen competes with gold-labeled antigen for antibody binding | Small molecule detection (hormones, drugs, toxins) | Drug screening, mycotoxins |
| Indirect Method | Gold-labeled secondary antibody detects primary antibody | Antibody detection | HIV antibody, COVID-19 antibody |
1.3 LFA Signal Types
| Signal Type | Label | Detection Method | Sensitivity | Advantages | Disadvantages |
|---|---|---|---|---|---|
| Colorimetric | Colloidal gold (20-40 nm) | Visual reading | nM-μM | No instrument needed, low cost | Semi-quantitative, subjective |
| Fluorescent | Fluorescent microspheres, quantum dots | Fluorescence reader | pM-nM | Quantitative, high sensitivity | Instrument needed, higher cost |
| Magnetic | Magnetic nanoparticles | Magnetic signal detection | pM-nM | Low background, quantitative | Specialized instrument needed |
| SERS | Gold nanoparticles + Raman probes | Raman spectroscopy | fM-pM | Ultra-high sensitivity | Expensive instrument |
II. Main Application Areas of LFA
| Application Area | Specific Test Items | Format | Signal Type | Clinical/Application Significance |
|---|---|---|---|---|
| Infectious Disease Rapid Testing | COVID-19 antigen/antibody, Influenza A/B, HIV, Dengue, Malaria | Sandwich/Indirect | Colloidal gold/Fluorescent | Rapid screening, primary care |
| Pregnancy and Fertility | HCG, LH | Sandwich | Colloidal gold | Home self-test, ovulation monitoring |
| Drug Abuse Screening | Morphine, Methamphetamine, Ketamine, Marijuana, Cocaine | Competitive | Colloidal gold | Drug control, public safety |
| Cardiac Markers | cTnI, NT-proBNP, D-dimer | Sandwich | Fluorescent | Rapid diagnosis of MI/heart failure |
| Inflammation Markers | CRP, PCT | Sandwich | Fluorescent | Infection differentiation |
| Food Safety | Clenbuterol, veterinary drug residues, mycotoxins | Competitive | Colloidal gold | Food safety regulation |
| Diabetes Management | HbA1c | Sandwich | Fluorescent | Diabetes monitoring |
| Veterinary Testing | Canine distemper, Feline panleukopenia, Heartworm, antibody testing | Sandwich/Indirect | Colloidal gold | Veterinary medicine |
III. Key Raw Materials and Solutions for LFA
3.1 Antigens and Antibodies — The "Recognition Core" of LFA
The quality of antibodies (or antigens) in LFA directly determines the sensitivity, specificity, and stability of the test strip. LFA has specific requirements for raw materials: high affinity (short reaction time), high specificity (avoid cross-reactivity), and good stability (heat-resistant, dry-resistant).
| Raw Material Name | Function | Recommended Specification | Selection Points |
|---|---|---|---|
| Gold-Labeled Antibody | Colloidal gold-antibody conjugate; binds to antigen during chromatography | Affinity ≥10⁸ L/mol; post-conjugation activity retention ≥70% | Conjugation process; purify to remove free antibody |
| Test Line Capture Antibody (T line antibody) | Captures antigen-gold-labeled antibody complex | Affinity ≥10⁸ L/mol, high specificity | Recognizes different epitope from gold-labeled antibody |
| Control Line Capture Antibody (C line antibody) | Captures unbound gold-labeled antibody (anti-gold-labeled antibody) | High affinity | Species-specific (anti-mouse/rabbit/goat IgG) |
| Antigen (Competitive Method) | Gold-labeled antigen competes with sample antigen for antibody binding | High purity, high activity | Labeling does not affect antigenicity |
| Blocking Agents/Antibodies | Eliminate heterophilic antibody interference | High activity | Improves specificity |
Special Requirements for Antibodies in LFA:
| Requirement | Reason | Recommendation |
|---|---|---|
| High Affinity | Reaction time is only 10-20 minutes | Affinity ≥10⁸ L/mol |
| High Specificity | No washing step; cross-reaction directly leads to false positives | Validated by cross-reactivity testing |
| Thermal Stability | Test strips stored at room temperature | Accelerated stability validation (45°C, 14 days) |
| Pairing Compatibility | Gold-labeled and capture antibodies must recognize different epitopes | Pair screening validation |
3.2 Colloidal Gold — The "Visualization Core" of LFA (Colorimetric Method)
Colloidal gold is the most classic signal label in LFA. It binds to antibodies through electrostatic adsorption or covalent coupling, producing visible red/purple-red bands on the test line.
| Raw Material Name | Function | Recommended Specification | Selection Points |
|---|---|---|---|
| Colloidal Gold Particles | Labels antibodies, produces visible signal | Particle size 20-40 nm, OD 520 nm | Good monodispersity, no aggregation |
| Colloidal Gold-Antibody Conjugate | Core detection reagent | Stable conjugation, no aggregation | Purify to remove free antibody |
| Gold Conjugate Storage Buffer | Maintains gold-labeled antibody stable and dispersed | pH 7.2-7.6, contains BSA, trehalose | Prevents aggregation and activity loss |
Relationship Between Colloidal Gold Particle Size and Performance:
| Particle Size | Color | Sensitivity | Stability | Application Scenario |
|---|---|---|---|---|
| 15-20 nm | Orange-red | High | Moderate | High sensitivity requirements |
| 20-30 nm | Red-purple | Medium | Good | Routine LFA (recommended) |
| 30-40 nm | Purple-red | Lower | Good | Clearer visual reading |
| 40-60 nm | Blue-purple | Low | Good | Special applications |
Colloidal Gold Preparation Method (Sodium Citrate Reduction Method):
| Step | Operation | Conditions | Explanation |
|---|---|---|---|
| 1. Heat | Heat 0.01% chloroauric acid solution to boiling | 100°C | — |
| 2. Reduce | Quickly add 1% sodium citrate solution | Stirring, maintain boiling | Color change: yellow → colorless → blue → red |
| 3. Cool | Stop heating, cool to room temperature | — | — |
| 4. Test | UV-Vis spectrum scan | 520 nm | Peak width should be ≤100 nm |
Colloidal Gold-Antibody Conjugation Process:
| Step | Operation | Conditions | Explanation |
|---|---|---|---|
| 1. Adjust pH | Adjust colloidal gold pH with 0.1 M K₂CO₃ | pH 8.0-8.5 | Near antibody isoelectric point |
| 2. Conjugate | Add antibody (5-15 μg/mL colloidal gold) | Room temperature, 30-60 minutes | Stir; antibody amount needs optimization |
| 3. Block | Add 10% BSA (final concentration 1%) | Room temperature, 15-30 minutes | Block unbound sites |
| 4. Centrifuge | Centrifuge at 4°C to remove free antibody | 8000-12000 rpm, 30-60 minutes | — |
| 5. Resuspend | Resuspend in gold conjugate storage buffer | Contains BSA, sucrose, trehalose | Store at 4°C |
3.3 Fluorescent Microspheres — The "Quantitative Core" of LFA (Fluorescent Method)
Fluorescent microspheres enable quantitative detection with sensitivity 2-3 orders of magnitude higher than colloidal gold, suitable for POCT scenarios requiring quantification.
| Raw Material Name | Function | Recommended Specification | Selection Points |
|---|---|---|---|
| Fluorescent Microspheres | Label antibodies, produce fluorescent signal | Size 100-300 nm, excitation/emission wavelengths | High fluorescence intensity, good monodispersity |
| Time-Resolved Fluorescent Microspheres | Eliminate background fluorescence interference | Europium (Eu³⁺) chelate-labeled | Higher sensitivity |
| Quantum Dots | Label antibodies, produce fluorescent signal | CdSe/ZnS core-shell structure | High fluorescence intensity, tunable spectrum |
3.4 Nitrocellulose (NC) Membrane — The "Reaction Platform" of LFA
The NC membrane is the most important consumable in LFA. It immobilizes the T line and C line antibodies and provides the channel for sample chromatography. NC membrane quality directly affects detection sensitivity, result clarity, and batch-to-batch consistency.
| Raw Material Name | Function | Recommended Specification | Selection Points |
|---|---|---|---|
| NC Membrane | Immobilizes T line/C line antibodies, provides chromatography channel | Pore size 5-15 μm, flow rate 40-120 seconds/4 cm | Moderate flow rate, high protein binding capacity |
| Backing Card | Supports NC membrane and other components | PVC material with pressure-sensitive adhesive | Moderate adhesion, flat |
Key Parameters of NC Membrane:
| Parameter | Recommended Range | Impact |
|---|---|---|
| Pore Size | 5-15 μm | Larger pore → faster flow → lower sensitivity; Smaller pore → slower flow → higher background |
| Flow Rate | 40-120 seconds/4 cm | Affects reaction time and sensitivity |
| Protein Binding Capacity | ≥30 μg/cm² | Affects antibody dispensing efficiency |
| Thickness | 100-200 μm | Affects mechanical strength |
3.5 Sample Pad and Conjugate Pad — The "Pre-treatment Zone" of LFA
| Raw Material Name | Material | Function | Selection Points |
|---|---|---|---|
| Sample Pad | Glass fiber, polyester fiber | Receives sample, filters particles, adjusts pH | Good absorbency, low protein adsorption |
| Conjugate Pad | Glass fiber | Pre-coated with gold-labeled/fluorescent-labeled antibodies | Good release properties, low protein adsorption |
| Absorbent Pad | Cellulose | Provides capillary driving force | Strong absorbency |
Sample Pad Treatment Process:
| Step | Operation | Function |
|---|---|---|
| 1. Prepare treatment buffer | PBS + BSA + Tween-20 + sugars + preservative | Provides stable environment |
| 2. Soak | Soak sample pad in treatment buffer | Uniform absorption |
| 3. Dry | Dry at 37°C for 2-4 hours | Remove moisture |
| 4. Store | Seal with desiccant | Moisture protection |
Conjugate Pad Treatment Process (Gold-Labeled Antibody Spraying):
| Step | Operation | Function |
|---|---|---|
| 1. Prepare gold conjugate solution | Dilute gold-labeled antibody to OD 5-10 | Add sugars, BSA, PVP |
| 2. Spray | Use sprayer to spray gold conjugate onto conjugate pad | Uniform spraying |
| 3. Dry | Dry at 37°C for 2-4 hours | Immobilize gold-labeled antibody |
| 4. Store | Seal with desiccant | Moisture protection |
3.6 Blocking Agents and Stabilizers
| Raw Material Name | CAS No. | Recommended Concentration | Main Application |
|---|---|---|---|
| BSA | 9048-46-8 | 1-3% | Block NC membrane, stabilize gold-labeled antibody |
| Casein | 9000-71-9 | 0.5-2% | Block NC membrane, reduce background |
| PVP K30 | 9003-39-8 | 1-2% | Stabilize colloidal gold, prevent aggregation |
| Sucrose | 57-50-1 | 5-10% | Protect antibody, promote release |
| Trehalose | 6138-23-4 | 2-10% | Protect antibody, thermal stability |
| Tween-20 | 9005-64-5 | 0.1-1% | Improve flow, reduce non-specific adsorption |
| Triton X-100 | 9036-19-5 | 0.1-0.5% | Lysis, improve flow |
NC Membrane Blocking and Stabilization:
| Step | Operation | Function |
|---|---|---|
| 1. Dispense | Dispense T line/C line antibodies | — |
| 2. Dry | Dry at 37°C for 1-2 hours | — |
| 3. Block | Immerse in buffer containing 0.5-1% BSA/casein | Block non-specific sites on NC membrane |
| 4. Dry | Dry at 37°C for 1-2 hours | — |
| 5. Post-treatment | Add sugars for protection | Improve stability |
IV. LFA Formulation Examples
4.1 NC Membrane Dispensing Formulation
| Component | Concentration | Function |
|---|---|---|
| Capture antibody (T line) | 0.5-2 mg/mL | Captures antigen-gold conjugate complex |
| Anti-gold-labeled antibody (C line) | 0.5-1 mg/mL | Captures unbound gold-labeled antibody |
| PBS buffer | 10-50 mM, pH 7.2-7.4 | Buffer environment |
| Sucrose | 5-10% | Protects antibody |
| Trehalose | 1-5% | Stabilizes antibody |
Dispensing Parameters:
| Parameter | Recommended Value |
|---|---|
| Dispensing volume | 0.5-1.5 μL/cm |
| Dispensing width | 0.5-1.0 mm |
| Line spacing | 4-8 mm |
4.2 Sample Pad Treatment Buffer Formulation
| Component | Concentration | Function |
|---|---|---|
| PBS buffer | 10-50 mM, pH 7.2-7.4 | Buffer environment |
| BSA | 0.5-2% | Reduces non-specific adsorption |
| Tween-20 | 0.1-0.5% | Improves flow |
| PVP K30 | 1-2% | Reduces non-specific adsorption |
| Sucrose | 5-10% | Protects antibody |
| ProClin 300 | 0.02-0.05% | Preservative |
4.3 Conjugate Pad Gold Conjugate Formulation
| Component | Concentration | Function |
|---|---|---|
| Gold-labeled antibody | OD 5-20 | Core detection reagent |
| Tris/HEPES buffer | 10-50 mM, pH 8.0 | Buffer environment |
| BSA | 1-2% | Stabilizes gold conjugate |
| Sucrose | 5-10% | Promotes release |
| Trehalose | 2-5% | Thermal stability |
| PVP K30 | 1-2% | Prevents aggregation |
| Tween-20 | 0.1-0.5% | Improves release |
4.4 Assembly Process
| Step | Operation | Explanation |
|---|---|---|
| 1. Backing card preparation | Remove release liner | Expose adhesive surface |
| 2. Attach NC membrane | Apply NC membrane to center of backing card | Correct orientation, align |
| 3. Attach conjugate pad | Apply conjugate pad overlapping NC membrane top (2-3 mm) | Ensure continuous chromatography |
| 4. Attach sample pad | Apply sample pad overlapping conjugate pad top (2-3 mm) | — |
| 5. Attach absorbent pad | Apply absorbent pad overlapping NC membrane bottom (2-3 mm) | — |
| 6. Press | Use roller to press all components together | Ensure tight contact |
| 7. Cut | Cut into 3-5 mm width strips | Sharp blade, clean cuts |
| 8. Insert into cassette | Insert into cassette (optional) | Moisture protection |
| 9. Package | Place in aluminum foil bag with desiccant, seal | Moisture-proof storage |
4.5 Test Strip Structure and Assembly Diagram
Structure:
[Sample Pad] → [Conjugate Pad] → [NC Membrane] → [Absorbent Pad]
↓ ↓
[T Line] [C Line]
(↓ Chromatography direction)
Assembly Overlap:
Sample Pad overlaps Conjugate Pad (2-3 mm)
Conjugate Pad overlaps NC Membrane (2-3 mm)
Absorbent Pad overlaps NC Membrane (2-3 mm)
V. LFA Procedure and Result Interpretation
| Step | Operation | Conditions | Explanation |
|---|---|---|---|
| 1. Apply sample | Add sample (50-100 μL) | Center of sample pad | Avoid bubbles |
| 2. Chromatography | Incubate at room temperature | 10-20 minutes | Wait for liquid to completely pass T line |
| 3. Read | Visual/fluorescence reader readout | Within specified time window | Delayed reading may cause false positives |
Result Interpretation Criteria:
| Result | T Line | C Line | Explanation |
|---|---|---|---|
| Positive | Colored | Colored | Target antigen present |
| Negative | No color | Colored | Target antigen absent |
| Invalid | No color | No color | Test strip invalid |
| Invalid | Colored | No color | C line antibody or gold conjugate issue |
VI. Frequently Asked Questions (FAQ)
Q1: Weak test line color in LFA test strip, how to solve?
A: Weak test line color is usually related to gold-labeled antibody, NC membrane, or antibody concentration.
| Cause | Solution |
|---|---|
| Low gold-labeled antibody activity | Optimize conjugation conditions; increase gold-labeled antibody OD |
| Low T line antibody concentration | Increase T line antibody dispensing concentration (1-3 mg/mL) |
| Poor NC membrane quality | Change NC membrane brand or lot; check pore size and flow rate |
| Too fast flow rate | Use NC membrane with smaller pore size; increase NC membrane length |
| Poor gold conjugate release | Optimize conjugate pad treatment buffer (increase sucrose to 10-15%) |
| Sample matrix interference | Optimize sample pad treatment buffer; add blocking agents |
Q2: High background or false positives in LFA test strip, what causes this?
A: High background or false positives are usually related to blocking, washing, or antibody specificity.
| Cause | Solution |
|---|---|
| Insufficient NC membrane blocking | Increase blocking agent concentration (BSA 1-3% or casein 0.5-2%) |
| Non-specific binding of gold-labeled antibody | Optimize gold conjugate purification; increase blocking agents (BSA, casein) |
| Cross-reactivity | Replace with higher specificity antibodies; add blocking agents |
| Reading time too long | Read within specified time window (10-20 minutes) |
| Improper sample pad treatment | Optimize sample pad treatment buffer (increase BSA, PVP) |
Q3: Abnormal flow rate in LFA test strip (too fast/too slow), how to solve?
A: Abnormal flow rate is related to NC membrane, sample pad, or conjugate pad.
| Cause | Solution |
|---|---|
| NC membrane pore size too large (too fast) | Use NC membrane with smaller pore size |
| NC membrane pore size too small (too slow) | Use NC membrane with larger pore size |
| Poor sample pad/conjugate pad release (too slow) | Increase sugar concentration in treatment buffer (sucrose, trehalose) |
| High sample viscosity (too slow) | Dilute sample; increase Tween-20 concentration |
| Test strip exposed to moisture | Check package seal; ensure thorough drying |
Q4: Poor room temperature storage stability of LFA test strips, how to improve?
A: Poor storage stability is usually related to drying process, formulation, or packaging.
| Cause | Solution |
|---|---|
| Incomplete drying | Extend drying time (37°C for 4 hours or vacuum drying) |
| Insufficient protectants | Increase sugar concentration (trehalose 5-10%, sucrose 5-10%) |
| Poor package seal | Use aluminum foil bag with desiccant; vacuum or heat seal |
| Gold-labeled antibody aggregation | Optimize storage buffer (increase BSA, PVP, trehalose) |
| Antibody detachment from NC membrane | Increase sugar concentration in dispensing buffer; use crosslinker (optional) |
Q5: Decreased gold-labeled antibody activity after conjugation, what to do?
A: Decreased activity after conjugation is usually related to pH, antibody amount, or protectants.
| Cause | Solution |
|---|---|
| Inappropriate conjugation pH | Adjust colloidal gold pH to near antibody isoelectric point (typically pH 8.0-8.5) |
| Insufficient antibody amount | Optimize minimum antibody protection amount (determine by NaCl challenge test) |
| Harsh centrifugation conditions | Reduce centrifugation speed (6000-8000 rpm) or shorten time |
| Improper storage buffer | Use storage buffer containing BSA and sugars (BSA 1-2%, sucrose 5% + trehalose 2%) |
| Lyophilization damage | Avoid lyophilization; or optimize lyoprotectants (trehalose 10-20%) |
Q6: How to improve detection sensitivity in fluorescent LFA?
A: Sensitivity can be improved from the following aspects.
| Aspect | Solution |
|---|---|
| Fluorescent microsphere selection | Use time-resolved fluorescent microspheres (europium chelate); select high-fluorescence intensity microspheres |
| Fluorescence reader | Use high-sensitivity PMT detector |
| Antibody affinity | Select high-affinity antibodies with affinity ≥10⁸ L/mol |
| Microsphere size | Use 200-300 nm microspheres (larger size gives stronger fluorescence) |
| Background control | Optimize NC membrane blocking; use low-fluorescence background NC membrane |
Q7: Can sodium azide be used in LFA test strips?
A: Not recommended. Although LFA does not depend on HRP activity like ELISA/CLIA, sodium azide still poses potential risks.
| Risk | Explanation |
|---|---|
| Affects antibody activity | Long-term exposure may affect antibody stability |
| Toxicity | Safety concerns for home self-test kits |
| Waste disposal | Increases disposal costs |
Recommended alternatives: ProClin 300 (0.02-0.05%) or Gentamicin (0.005-0.01%)
VII. Summary
LFA, as the core technology platform in the POCT field, has test strip performance that heavily depends on the proper selection and combination of antibody pairs, colloidal gold/fluorescent microspheres, NC membranes, sample pads, conjugate pads, blocking agents, stabilizers, and other raw materials.
艳 李
