What Causes Invalid Results in Nasal Swab-Based Respiratory Infection Rapid Tests?

May 25, 2026 Editorial Reszon

Invalid results may occasionally occur in lateral flow rapid antigen testing when testing procedures are not performed correctly.

In most cases, the absence of a control (C) line indicates that the test flow was interrupted due to sample handling or procedural factors rather than product performance itself.

This technical guide highlights common factors that may affect assay validity and provides practical tips to support accurate testing for nasal swab samples.

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Understanding the Common Factors Leading to Invalid Result

The “Traffic Jam”: Sample is Too Thick We often think “more mucus means a better sample”, but the test strip is a delicate piece of paper. Excessively viscous or mucus-heavy samples may impair liquid migration along the test strip.	What Happens: The liquid cannot travel down the strip. If it fails to reach the Control (C) area, the test cannot run correctly and resulting in a blank or invalid result window.
The “Dry Run”: Not Enough Sample Rapid tests are designed to operate with a specific sample volume, typically 2-3 drops (Please refer to manufacturer’s instruction for use).	What Happens: Adding too few drops, or accidentally dropping liquid sample outside the sample well, can disrupt the test process. Without sufficient liquid, the sample cannot properly “wick” along the test strip. As a result, it never reaches the control (C) line, leading to an invalid result. Think of it like a marathon runner who runs out of water halfway through the race—the finish line is there, but they simply can’t reach it.
The “Flood”: Too Much Sample It’s tempting to think that “more is better,” but adding extra drops of sample actually disrupts the capillary flow mechanism of the assay.	What Happens: The test strip is designed with INVALID a specific “liquid capacity.” and rely on capillary action – the same way a paper towel absorbs a spill. When the sample well is flooded, the strip becomes oversaturated. Instead of a smooth, controlled flow, the liquid pools on the strip, disrupts the chemistry and preventing the test lines from forming correctly.
The “Impatience” Factor: Reading Too Early When it comes to rapid tests, timing is critical. Checking the result before the recommended time (usually 15-20 minutes) is a common cause of invalid or false-negative readings.	What Happens: At 5 or 10 minutes, the reaction may still be developing. Reading the test before the recommended time may result in incomplete assay development and inaccurate interpretation.

How to Ensure a Valid Result Next Time

Step	What To Do	Why It Matters
The Swirl	Rotate the swab in the extraction buffer for at least 10 times or 15–30 seconds.	Releases the virus from the swab into the extraction buffer.
The Squeeze	Squeeze the sides of the tube against the swab as you pull it out.	Helps maximize recovery of viral antigens from the swab.
The Count	Hold the tube vertically and count the drops exactly.	Helps maintain optimal sample flow across the test strip.

Best Practices for Performing an Accurate Lateral Flow Assay

1. The “Perfect Drop”:
Always hold the sample tube completely vertical (straight up and down) over the sample well. This ensures each drop is the correct volume. Aim directly for the center of the sample well!

2. The “Squeeze & Swirl”:

Swirl the swab vigorously in the extraction buffer, then squeeze the sides
of the tube against the swab tip as you pull the swab out. This helps release concentrated viral proteins needed for a better signal.

3. The “Flat & Time” Rule:
Keep the test cassette on a flat surface and do not move or tilt it while the liquid is flowing. Set a timer and read the result exactly at the time specified – no earlier and no later.

4. Follow Manufacturer’s Instruction:
For best results, always read instruction for use and watch the manufacturer’s video guide before performing the test. The step-by-step video demonstrates correct swab handling, sample preparation, sample drop counting and result interpretation – helping you avoid common mistakes that lead to invalid results.

Example:
A video guide demonstrating how to use the RESZON ABC Combo Test – Flu A+B & COVID-19 Ag (Nasal Swab) is available here:

Quick Tips

Always place the test cassette on a flat surface. Tilting or lifting it while the test is running can cause uneven liquid flow, leading to streaks or invalid results.

Important Notes

If your test result is invalid, discard the test and repeat the process with a new swab and a new test cassette.
If your result is negative but symptoms persist or worsen, consult a healthcare professional, as early-stage infections may not always be detected.