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Decoding the Importance of Waste Pads in Lateral Flow Devices

Introduction

In the rapidly evolving field of diagnostics, lateral flow tests have carved a niche for themselves as a reliable tool for quick results. While the spotlight often shines on the more visible components such as the test and control lines or the cassette, there exists a key player that silently contributes to the test's functionality - the waste pad. This blog post aims to shed light on the importance of the waste pad and how its selection can significantly influence the performance of a lateral flow test.

The Role of Waste Pads: More Than Meets the Eye

The waste pad, also referred to as the wick(ing) or absorbent pad, is positioned at the end of the lateral flow strip – opposite of the sample application area. Its main role is to serve as a take-up reservoir for the reagents. Without an absorbent pad, the flow of the test would cease as soon as the liquid front has reached the far end of the reaction membrane. In this case the sample volume that will give information about analyte amount is very limited (about only the void volume of the membrane) – and therefore, also the sensitivity of your test might be very limited. By incorporating an absorbent pad, this volume can be substantially increased and thus also the sensitivity of your assay is increased by simply moving a higher amount of analyte over the test line zone.
An interesting effect is observable when the membrane begins to dry after the assay has concluded: reversal of flow direction. Liquid in the absorbent pad flows back onto the membrane This can lead to false positive results when the assay is read too late.

A seemingly straightforward component is instrumental in maintaining the test's precision and reliability by managing the flow of and preventing backflow.

“A good way to terminate a lateral flow run: remove the waste pad. Although only feasible for dipstick tests, i.e. LFT without cassette.”

 

Key Factors to Consider When Selecting a Waste Pad

• Absorption Capacity: The waste pad should possess a high absorption capacity to effectively soak up the surplus sample and buffer. If the pad's absorption capacity is inadequate, it could lead to backflow, disrupting the test's flow and potentially leading to inaccurate results. Absorption capacity is manly driven by:
• • thickness of the material,
  • size of the pad on the strip and
  • the absorption properties of the material.

So generally speaking: A longer waste pad from the same material will increase the total absorption capacity of your LFT.

• Material: Waste pads are typically crafted from cellulose or polyester. An interesting alternative are polyolefin based materials (for example by POREX). The choice of material can impact the pad's absorption capacity and the rate of flow. It's essential to choose a material that is compatible with the other components of the test. Especially, when you are using a cassette you need to find a material that fits thickness-wise into your housing.
• Size: The size of the waste pad should be sufficient to absorb the volume of the sample and buffer used in the test. A pad that is too small may not absorb all the surplus liquid, while a pad that is too large may unnecessarily increase the size and cost of your test. On the supplier pages you generally find information about water absorption per area of the specific materials to guide your decision-

 

Typical Issues of Waste Pad Selection on Test Performance

The selection of a waste pad can influence the performance of a lateral flow test. A high-suitable waste pad ensures a smooth and consistent flow, which is vital for the formation of the test and control lines.

• Insufficient capacity: see above.

• Irregular or slow flow: This can be caused when nitrocellulose and waste pad are not sufficiently overlapping. Another reason could be compression of the material. This can be found when the dimensions of the cassette and the strip materials are not matching.

• Poor quality: Contamination of the material during manufacturing or storage.

 

 

Typical Suppliers

 

Here you can find a short list of starting points for your waste pad research:

• Ahlstrom:

https://www.ahlstrom.com/products/medical-life-sciences-and-laboratory/lateral-flow-test-pads/absorbent-pads/

• Cytiva

https://www.cytivalifesciences.com/en/us/solutions/lab-filtration/knowledge-center/absorption-pads

• MDI

https://mdimembrane.com/absorbent-pads#:~:text=Absorbent%20Pads%20Absorbent%20pads%2C%20located%20at%20downstream%20end,pads%20for%20dipstick%20as%20well%20as%20device%20formats

• Porex – ohne Glasfaser und Zellulose

https://www.porex.com/life-sciences/diagnostics/lateral-flow-assays/absorbent-pads/

Conclusion

While the waste pad may seem like a minor component in a lateral flow test, its role is far from minor. The selection of a waste pad can significantly influence the test's performance, underlining the importance of careful selection and quality control in lateral flow test development.

At Steven Steiner Consulting, I comprehend the complexity of lateral flow test development. With my expertise in diagnostics and a keen eye for detail, I can assist you in understanding these complexities and developing high-quality, reliable lateral flow tests. Reach out to me today to learn more about my consulting services.

 

If you need further assistance or consultation, don't hesitate to reach out to me.

Don’t hesitate to contact me and book a meeting.

Share your own experiences, ideas, or challenges related to lateral flow test cassettes and send me a message.

 

Disclaimer: All Pictures and Sketches were produced by AI Bing Image Creator.

References

 

[1] https://www.sigmaaldrich.com/DE/de/technical-documents/technical-article/clinical-testing-and-diagnostics-manufacturing/ivd-manufacturing/pads-chemistries-selections-specifications-and-conjugates

[2] https://www.technologynetworks.com/diagnostics/articles/an-introduction-to-the-lateral-flow-test-strengths-limitations-and-applications-370382

[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7119943/

[4] https://www.emdmillipore.com/INTERSHOP/static/WFS/Merck-Site/-/Merck/en_US/Freestyle/DIV-Divisional/Events/pdfs/lateral-flow-presentations/troubleshooting-lateral-flow-tests.pdf