App note

Detecting Proteins Directly from Whole Blood

A small plasma-separation chip turns a few drops of blood into clean plasma in about 15 minutes, with no centrifuge, and feeds it straight to the P4SPR.

SystemP4SPR

ApplicationPoint-of-care diagnostics, serology

SampleWhole blood

In short: Researchers at the University of Waterloo, working with Affinité, coupled a passive plasma-separation chip to the P4SPR. From 160 µL of whole blood, the chip gives about 22 µL of plasma in 15 minutes, and the P4SPR detects antibodies in it at levels far below those normally found in blood.

The need

Blood tests start with a centrifuge: red cells scatter light, clog channels and add background, so plasma has to be separated first. That step needs lab equipment and a trained person, which is exactly what is missing at the bedside, in a clinic or in the field. A test that starts from a finger-prick volume of whole blood removes it.

How it works

Whole blood goes into the inlet of a small polymer chip. Red cells settle and are held back by a membrane, and capillary forces pull clear plasma into a collection well; nothing is pumped or spun. The plasma then goes to the P4SPR, where an AffiCoat sensor carries antibodies against the protein of interest. Binding shows up in real time, label-free (Figure 1).

Whole blood into a separation chip, red cells held back, clear plasma onto the SPR sensor where antibodies capture the target — Figure 1. From blood to binding: whole blood in, red cells held back by the chip, plasma onto the antibody-coated sensor.

What the study showed

	Result
Blood in, plasma out	About 22 µL of plasma from 160 µL of whole blood in 15 minutes; no centrifuge, no pump
Detection in whole blood	Human IgG and IgM spiked into blood were detected down to the low nanomolar range, far below their normal blood levels
Specificity	Anti-IgG and anti-IgM surfaces each responded most to their own target

Full method and data: Debnath, Live and Poudineh, Lab on a Chip 2023 (reference below).

Why the P4SPR fits point-of-care work

What a point-of-care test needs	What the P4SPR brings
Small and portable	Benchtop, USB-powered, no moving parts
Open to new sample prep	The fluidic cell can be adapted: here, a custom four-channel chip fed by syringes
Low volumes	Microlitres of plasma are enough for a measurement
Complex samples	AffiCoat low-fouling surface keeps plasma background down
Several markers at once	Four channels, read in turn: one antibody per channel, or a reference channel

Beyond IgG and IgM

The capture antibody sets the target. Swap it, and the same blood-to-plasma-to-sensor chain applies to other serological markers: antibodies against viral or bacterial antigens, or protein biomarkers. For a clinical example in plasma, see our note on screening plasma for IgA deficiency.

What you need

Item	Source
P4SPR	Affinité Instruments
AffiCoat Sensor (SAMAC-01) and Amine Coupling Kit (AMIN-KIT)	Affinité Instruments
Capture antibodies (here: anti-human IgG, anti-human IgM)	Commercial
Plasma separation chip	Design described in the reference below
Running buffer with BSA and surfactant	Standard reagents

Reference

N. Debnath, L. S. Live, M. Poudineh, "A microfluidic plasma separation device combined with surface plasmon resonance for biomarker detection in whole blood," Lab on a Chip, 2023. doi:10.1039/D2LC00693F.

Illustrations by Affinité Instruments.