Membranes 101: A guide to membranes for POC diagnostics

Point-of-care (POC) diagnostics are rapid tests used to detect a wide range of analytes from various biological samples. Use this guide on membrane selection to optimize your POC tests for maximum accuracy, sensitivity, and efficiency.

What are point-of-care (POC) diagnostic assays?

In recent years, POC testing has revolutionized healthcare. By bringing “the lab” to the patient, these tests provide inexpensive, simple-to-perform diagnostics that can deliver a rapid turnaround of results at the site of care delivery.

POC diagnostics take a variety of modalities and forms. Ranging from basic test strips to sophisticated molecular POC assays, each platform needs to be optimized for the sample type analyzed and designed to obtain a rapid result from minimal sample input.

In general, POC assays fit into two broad categories: POC immunoassays and molecular POC assays. Immunoassays most commonly use antibodies to detect the chosen analyte, which can include antigens, drug molecules, or proteins. In contrast, molecular POC assays detect nucleic acids, such as DNA or RNA, present within a sample.

Together, POC immunoassays and molecular POC assays are increasingly used across healthcare services to diagnose a wide range of conditions, including infectious diseases, cancer, and more.

Why is the choice of membrane so important when developing a POC test? 

The choice of membranes and pads used in POC tests is central to their success. They act as filters that facilitate sample transport, analyte separation, reagent immobilization, and flow control. These functions influence key aspects of the test’s performance, including sensitivity, specificity, and speed.

With your choice of membrane being key to successfully developing a POC test, this guide will cover our top tips and essential considerations to help you choose the optimal membranes for your POC immunoassays and molecular assays.

POC immunoassays

Some of the most frequently used POC immunoassays are lateral-flow tests (LFTs), flow-through immunoassays, and dipstick colorimetric assays. Each uses a different approach for its antibody-antigen detection, requiring a specialized set of membranes and pads.

Lateral-flow tests (LFTs)

Fig 1. LFT membrane and pad components

LFTs are single-step assays used to detect target molecules in a variety of analytes and are well known for their use in COVID-19 testing. This type of POC immunoassay requires several specialized pads and a central, porous membrane.

The sample pad is the first membrane encountered by the sample during a LFT. Sample pads are usually made from low- to medium-weight cotton linter membranes, which offer fast flow rates with high loading capacity and the ability to retain very fine particles.

Thinner cotton linter membranes, including CF1, are most suitable for smaller sample volumes. Cotton linter membranes with medium thickness, such as CF4, are more suitable for samples requiring increased wet strength and reduced trace impurity content.

When analyzing blood or serum samples, blood separators may also be used as a sample pad. Glass fiber sample pads are well suited for use as blood separators because they don’t affect red cell hemolysis, and red blood cells can effectively wrap around their fibers. When choosing a blood sample pad, thickness should be considered, and the optimal thickness will depend on the volume of blood required for your assay. An alternative approach is to use asymmetric membranes such as Vivid™ plasma separation media (PSM). These membranes do not exhibit hemolysis of the blood sample, which can be a restriction when using glass fibers. Vivid™ PSM have a much better plasma product efficiency than glass (i.e., for the same amount of blood added, you get more plasma from Vivid™ PSM). Vivid™ PSM are also easier to fabricate into devices compared to glass fibers. Over-compression of glass fibers causes them to crack, which increases the chance of sample hemolysis.

View our blog: Guidance for sample pad selection

Conjugate release pads are the second step of LFTs. They hold and preserve the detection reagents or conjugate. Glass fiber is a suitable choice for conjugate release pads because they do not require pretreatment and have consistent absorbency and wicking rates. Key considerations when selecting a glass fiber conjugate release pad include the water absorption capacity, sample retention, thickness, wicking rate, and conjugate release rate, required for your assay.

View our blog: Conjugate release pads for lateral flow immunoassays

Nitrocellulose membranes are the ubiquitous choice for LFTs and are used to bind and detect the target molecule. Combining rapid capillary flow with high binding capacity, nitrocellulose strikes an excellent balance between flow rate and assay sensitivity. Many types of nitrocellulose membranes exist, with different speed variants offered for almost any sample type (blood, urine, saliva, etc.). When deciding on the nitrocellulose membrane for your POC diagnostic, capillary flow time, sample viscosity, and membrane surfactants are essential considerations. Nitrocellulose membranes are well-liked for their liquid flowing characteristics, but they can provide challenges because natural variations in the membranes cause inconsistencies with flow rates, protein binding, and background. Another limitation of nitrocellulose membranes is that they have a finite lifespan, which can often be one of the shortest of all components in a test. The short lifetime will limit the shelf life claimed for a finished in-vitro diagnostic (IVD) test.

Read our blog on nitrocellulose membrane selection for lateral flow immunoassays

The absorption pad is the final test component in lateral-flow immunoassays, and its primary function is to act as a sink for the reagents. For this pad, cotton linter and glass fiber membranes may be used because these high-quality materials offer consistent absorbency, high sensitivity, rapid rewetting, and minimal leakage along the strip. When choosing the cotton linter or glass fiber membrane for your lateral flow assay absorption pad, thickness, wicking rate, and water absorption are key considerations.

Learn more about the considerations for lateral flow immunoassay absorption pads

Flow-through assays

Fig 2.Schematic of a flow-through assay

Flow-through assays work in a similar way to a standard enzyme-linked immunosorbent assay (ELISA). The reaction membrane is laid on top of an absorbent pad, which draws the samples and reagents through the membrane.

Nitrocellulose membranes are most frequently used for flow-through assays, due to their excellent wettability, binding capacity, easy blocking, and compatibility with a variety of analytes. Pore size and backing are key considerations when selecting nitrocellulose membranes for flow-through immunoassays. Small pore structures offer nonspecific binding and increased sensitivity, with unbacked membranes being suitable for most flow-through immunoassays depending on the sample’s mechanical load.

For absorption pads, thicker cellulose membranes are more suitable, providing fast wicking and high water-absorption capacity. Absorption capacity of the pad is arguably the most important consideration, with the pad needing to be in close contact with the entire lower surface of the reaction membrane to ensure a homogeneous flow and provide a consistent signal.

Read more on the considerations for flow-through immunoassay membrane selection

Dipstick assays

Fig 3.Example of a dipstick colorimetric assay

Dipstick assays are simple and cost-effective POC tests. These rely on the vertical flow of liquid samples through a porous membrane impregnated with dried reagent pads that interact with the analyte(s) on exposure.

Dipstick test pads demand membranes with high wettability, mechanical stability, consistent absorption properties, and rapid flow characteristics. The most important consideration for dipstick test pads is absorption capacity. Therefore, cotton linter membranes are usually recommended for this type of assay because they exhibit excellent absorption capacity, which allows the pad to handle a large sample volume without causing the test strip to fail or decompose from wet mechanical stress.

Find more advice on membrane selection for dipstick colorimetric assays

Molecular POC assays

In recent years, several molecular POC assays have been developed including LFTs and nucleic acid amplification tests (NAATs), which use various nucleic acid amplification technologies. Across many of these tests, nucleic acid isolation and cell isolation steps are central to their functionality, which can be facilitated via the use of membranes.

Nucleic acid isolation

Numerous membrane types can be used for nucleic acid isolation. Glass fiber membranes are especially popular, due to their high nucleic acid binding capacity and high wet strength compared to other membranes such as cotton linter. However, the optimal membrane for your application will depend on several characteristics such as the required thickness, pore size, porosity, and surface quality.

View our blog: Glass fiber filters for straightforward nucleic acid extraction

Cell isolation

Cell isolation steps are necessary for various molecular POC assays and require a membrane with highly defined pore sizes to effectively isolate the specific target cells. For this, track-etched membranes are a suitable option. These specialized membranes are manufactured using a precise etching process to create uniform and cylindrical pores, resulting in a well-defined porous structure that allows for accurate particle separation.

POC diagnostics membranes for your sample type and application

Here is an overview of commonly used membranes in POC immunoassays and molecular assays including the different sample and pad types they’re most suitable for.

Custom POC diagnostics services

Membrane selection and implementation for POC diagnostics can be a challenging process due to the many variables referenced above. However, selecting optimal membranes and pads is key to the success of your assay development.

Despite the numerous membrane options available, you may need an element of your POC test to be optimized further for your specific application. Our diagnostic services can customize a broad range of pads and membranes to suit your highly specific POC assay requirements. Customization options include slitting, cutting, punching, and a range of post treatment options.

If you are looking to speed up the commercialization of your assay and to collaborate with a leader in diagnostic development, Cytiva can provide immediate access to specialist knowledge and equipment without upfront investment. We can help you accelerate the translation of your innovative diagnostic concepts into reliable and effective POC solutions. To find out more about our custom diagnostic services, speak with a specialist today.