Membrane Coating & Lot-to-Lot Reproducibility¶
The T-line and C-line are where the quantitative signal is captured. Their physical uniformity — line width, reagent volume, and edge definition — is the single largest manufacturing determinant of assay precision (CV%). This page explains the coating physics and how reproducibility is achieved.
The Coating Problem¶
A capture reagent is dispensed as a continuous line onto the nitrocellulose membrane. For the assay to be quantitative and reproducible, the amount of capture reagent per unit length must be constant across the entire membrane and identical between lots.
Factors That Affect Coating Uniformity¶
| Factor | Effect on the line |
|---|---|
| Dispense rate (µL/cm) | Total reagent loading — drives signal intensity |
| Line width | Reagent density per unit area |
| Reagent viscosity | Flow consistency through the dispenser |
| Membrane surface energy | Line spreading / wicking |
| Temperature & humidity | Evaporation rate, line sharpness |
The Drying Step¶
After dispensing, the membrane is dried under controlled temperature and humidity. Drying is not a passive step:
- Too fast → uneven reagent distribution, "coffee-ring" effect, degraded activity.
- Too slow → reagent diffusion, blurred lines, high background.
- Incomplete → residual moisture degrades stability over shelf life.
How Lot-to-Lot Reproducibility Is Controlled¶
| Control | Method | Target |
|---|---|---|
| Dispense calibration | Gravimetric verification of dispense rate | Fixed µL/cm |
| Line-width monitoring | Optical measurement of line width | Within tolerance |
| Signal uniformity check | Fluorescence scan along the line | Low variance (→ low CV%) |
| Environment lock | Temperature/humidity control | Stable drying kinetics |
| Reference-lot comparison | New lot vs. reference curve | Within acceptance range |
Why This Produces Tight CV%¶
Precision (CV%) is the variance of signal across strips and lots. By holding dispense rate, line geometry, and drying constant, the dominant sources of variance are removed — leaving CV% below 10% as a process property rather than a fortunate outcome.
The Link to Analytical Validation¶
The analytical performance documented in Validation Methodology — LOD, CV%, linearity — is achievable in routine production only because the membrane coating process is this reproducible. See also the Lot-to-Lot Consistency Report.
FAQ¶
How is line uniformity verified? By optical line-width measurement and fluorescence-intensity scanning along the dispensed line, before the membrane is cut into strips.
What is the most common cause of high CV%? Non-uniform membrane coating — which is why it is the most tightly controlled step.
For the equipment used in coating, see the Knowledge Base.
Authored by: Migibio Clinical & Scientific Affairs, Guangzhou Magic Biotech Co., Ltd.
Reviewed by: Migibio R&D Quality Committee
Last updated: 2026-08-14
Disclosure: Migibio (Guangzhou Magic Biotech Co., Ltd.) is the manufacturer of the FIA680/FIA880 analyzers and FICT reagents referenced in this content. See our Editorial & Review Policy.