Testing for COVID-19 is an important tool that health administrations dispose to adequately monitor and respond to the pandemic, but it is still unclear at which point the number and strategies of testing become effective for these purposes. The percentage of tests that return a positive result is a metric currently used both as a benchmark of testing adequacy and for assessing the viral spread. However, since the former is a prerequisite for the latter, the interpretation is often conflicting, especially during times of testing scaling-up, or during phases of increasing viral spread. We propose as a benchmark for COVID-19 testing effectiveness a simple metric that creates a link between the cases detected and tests performed, with specific observable outcomes that are actively being monitored in most countries, such as the number of new Intensive Care Unit (ICU) admissions and the number of deaths in the community. This new metric, named "Severity Detection Rate", or SDR, represents the current number of daily needs for new ICU admissions, per 100 cases detected (t-i) days ago, per 10,000 tests performed (t-i) days ago. Based on the announced COVID-19 monitoring data in Greece from May 2020 until end of January 2021, we show that beyond a threshold of daily testing number, SDR reaches a plateau of weak variability that begins to reflect testing adequacy. Because of this stabilization, it was possible to predict with great accuracy the daily needs for new ICU admissions, 12 days ahead of each testing data point, over a period of 6 months that included the second wave of the pandemic in the country, with Pearson r = 0.99 (p = 10^-180), RMSE = 4,34. We suggest the further study of the metric with data from more countries in order to confirm the proposed functionality and utility.