Comparative performance of surrogate and specific tests for
SARS-CoV2
The key risk in using any POCT is of a false-negative result leading to
inappropriate management of SARS-CoV2 infection. For FebriDx, an
additional risk is that a positive result cannot exclude the possibility
of another virus as the cause of infection.
Studies evaluating FebriDx mostly consider performance in distinguishing
viral from bacterial causes of acute respiratory illnesses in
toto , in secondary care. Reported sensitivities range from 64-90%, and
specificities from 78-88% (13–16). However, two recent studies from
the UK evaluate FebriDx specifically for screening for SARS-CoV2 in
hospital: a small-scale pilot (5), and a study nested within a
non-randomised clinical trial of molecular POCTs (6). These studies
report impressive sensitivities and specificities of 100% and 93%, and
100% and 86% respectively. However, interpretation is limited by
[i] the use of clinical diagnosis as reference standard rather than
RT-PCR in the first study, [ii] evaluation in single secondary care
centres in England in both cases, and [iii] the inclusion of
patients in the range 2-7 days from symptom onset only in the first
study. Finally, test performance in both studies may have been
artificially boosted because they were conducted at times when the range
of co-circulating respiratory viruses was lower than in the autumn and
winter.
A comprehensive assessment of POCT field performance is beyond the scope
of this paper, and available data indicate large context-dependent
variations even for the same platform. However, based on data covering
the first 14 days from symptom onset collated by FIND (17), SARS
Co-V2-specific tests perform comparably to FebriDx (figure 1). Caveats
to this assessment are that: (i) the majority of tests are
antibody-based and therefore only reactive some time after symptom
onset, and (ii) most studies used patient samples collected in clinical
settings only. Evidence on the performance of FebriDx or any other POCT
for diagnosis of SARS-CoV2 infection in key workers, care home
residents, or other high-risk populations is in very short supply.