Timely estimation of the current value for COVID-19 reproduction factor $R$
has become a key aim of efforts to inform management strategies. $R$ is an
important metric used by policy-makers in setting mitigation levels and is also
important for accurate modelling of epidemic progression. This brief paper
introduces a method for estimating $R$ from biased case testing data. Using
testing data, rather than hospitalisation or death data, provides a much
earlier metric along the symptomatic progression scale. This can be hugely
important when fighting the exponential nature of an epidemic. We develop a
practical estimator and apply it to Scottish case testing data to infer a
current (20 May 2020) $R$ value of $0.74$ with $95\%$ confidence interval
$[0.48 - 0.86]$.