Point-of-Care Diagnostics - Holographic Sensors - page 3

diagnostics, WHO has outlined a set of criteria corresponding to the acronym
ASSURED: Affordable, Sensitive, Speci c, User-friendly, Rapid and Robust,

Equipment free and Deliverable to those who need it [ 10 ]. For the developing
world, the trend towards low-cost is a priority; however, clinically useful sensitivity
and speci

city from rapid diagnostics assays are also required. Operating with low-
volume samples without manual manipulation, being portable and functioning
without sharps such as needles are considered desirable characteristics of rapid
assays. Hence, the development of robust diagnostic tests can enable individuals
and local communities to monitor their health condition and timely mitigate the
spread of diseases.

1.1 The Development of Rapid Diagnostics

Historical assay formats for point-of-care testing are dipsticks and lateral fl owdevices.

In the 1950s, the earliest paper-based strip test emerged for the quanti cation of the

concentration of glucose in urine, and this product was marketed in the 1960s [ 26 ].
The principle of operation of the assay was based on glucose oxidase, and developed
colours were compared to a reference chart for interpretation. Today, commercial
urinalysis strip tests are adapted for a wide range of analytes. Traditionally, these tests
are semi-quantitatively read with a colorimetric chart or with detection equipment
such as CLINITEK Status ® + Analyzer (Siemens) or Urisys 1100 ® Urine Analyzer
(Roche). In the 1950s, parallel to the development of strip tests, latex agglutination
assays and radioimmunoassays were also developed [ 27 , 28 ]. Since the 1970s,
nitrocellulose matrices have been used as a substrate for molecular detection [ 29

31 ].

-

In the 1980s, serological lateral- ow tests emerged [ 32 ]. The most notable example is

the human pregnancy test, which was derived from the development of hCG beta-
subunit radioimmunoassay [ 33 ]. Since then, commercial rapid lateral

fl

ow assays
expanded beyond clinical diagnostics to veterinary, food, environmental applications,
bio-defence and drug abuse (Table 1.1 ). Rapid diagnostic tests often have lower
speci

fl

city and sensitivity than their laboratory bench counterparts. The majority of
these tests are simple and provide yes/no answer, where response time is critical to the
user.
The commercial rapid tests have various geometries and con gurations with/out

housing units (Fig. 1.1 ). Figure 1.1 d shows a multiplexed lateral-

fl ow assay, which

allows the analyses of a single sample simultaneously. Lateral- fl ow immunoassays

have two major con gurations: Direct (i.e. double antibody sandwich assays) and

competitive (i.e. inhibitive) formats. The assay format typically consists of a number
of segments: sample pad, conjugate pad, reaction membrane and an absorbent pad
(Fig. 1.1 e). These segments are supported by a backing card and enclosed in a plastic
cassette (housing). Another format in rapid diagnostics is

fl flow-through (vertical),

which is relatively more complex than lateral flow format, and its execution requires

(i) sample placement, (ii) washing and (iii) addition of analyte-colloidal gold
conjugates [ 68 ] (Fig. 1.1 f). 1.1f).Most of the commercial assays require sample preparation.

fl