Biomedical Engineering Reference
In the following, we will discuss various approaches to resolve the drawbacks of
conventional flow cytometers regarding their use for POC diagnostics.
Miniaturized Flow Cytometers and Microfluidic-Based
In recent years, a number of scaled-down “high-end” flow cytometer instruments
and microfluidic-based devices for POC diagnostics have been developed with the
promise of portability and reduced cost. The main driver for this development is the
urgent need to perform CD4 T-lymphocyte counts in resource-limited settings. This
is required for screening, initiation of treatment, and monitoring of HIV-infected
patients [ 5 ].
Flow-based analysis with instruments such as the FACSCount TM (Becton
Dickinson), EPICS XL/MCL TM (Beckman Coulter), Guava EasyCD4 TM (Milli-
pore/Merck), PointCare NOW TM (PointCare Technologies), or CyFlow CD4 TM
(Partec GmbH) is the established method for CD4 counting. However, these are
still quite expensive and sophisticated instruments, requiring a lab environment and
skilled operators. In addition to the established instruments, there are several under
development or have recently entered the market that are especially designed for
POC testing (e.g., from Axxin Ltd., Alere, Daktari Diagnostics, mBio Diagnostics,
Partec and Zyomyx). These instruments are discussed in greater detail in a recent
review article by Boyle et al. [ 6 ]. They are fast, quite robust, use a disposable
cartridge, and need only a small amount of analyte (e.g., finger prick of whole
blood). From this list, the recently introduced CyFlow miniPOC from Partec is the
only instrument that represents a miniaturized flow cytometer, tailored to the needs
of CD4 counting in the field. The others use different detection schemes to provide
the cell count.
The CD4 test reader from Axxin measures the concentration of a cell-associated
CD4 protein in whole blood rather than actually counting the CD4 cells. The
PIMA TM instrument from Alere is based on cell capturing, microfluidic sample
processing, and digital dual-color fluorescence image analysis. The device under
development by Daktari Diagnostics uses specific cell capturing in a microfluidic
cartridge functionalized with CD4 antibody. Differences in binding affinity and
effective shear forces are used to differentiate between lymphocytes and monocytes.
The CD4 count is determined by measuring the impedance change induced by the
released ions after lysing the captured CD4 cells. The SnapCount TM instrument
developed by mBio Diagnostics is based on immunostaining and cell capturing in
combination with a two-color fluorescence imaging system. The instrument-free
CD4 counting device developed by Zyomyx uses a sedimentation technique based
on functionalized magnetic beads. The CD4 count can be determined by eye from a
scale reading on the tubing.
Besides the commercial developments discussed above, there are many very
promising concepts and technologies for POC diagnostics discussed in the literature.