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Corona reference measurement procedure

Detecting SARS-CoV-2 by means of digital PCR

PTBnews 1.2021
Especially interesting for

medical laboratories

metrology institutes

physicians and patients

Droplet digital PCR has been used in international comparison measurements to ensure the quality of SARSCoV- 2 tests for the first time. Hereby, this method proved particularly successful to quantify the RNA concentration.

Multichannel pipette while filling a multiwell plate (large picture) and scheme showing how fragments of nucleic acid are digitized in a droplet digital polymerase chain reaction (ddPCR) (inset). Here, digitizing means that each viral RNA fragment that has been found is assigned the value 1.

The SARS-CoV-2 pandemic has shown that tests evaluated by medical laboratories are of paramount importance in detecting an infection. The term PCR has now become familiar to most people. The test methods are based on detecting viral RNA using PCR (polymerase chain reaction) methods. These methods involve recognizing certain sequences of the viral genome and amplifying them highly specifically by means of PCR until they can be detected. Hereby, a positive test result will only be obtained if the viral RNA that is being searched for is really present.

For these tests, it is extremely important to know the reliability, comparability and reproducibility of the test results. To ensure the quality and comparability of the test results, PTB is developing, among other things, reference measurement procedures to detect the viral genome of SARS-CoV-2. The contribution these reference measurement procedures can provide has now been investigated within the scope of international comparison measurements between medical laboratories. These comparisons were organized by INSTAND e. V. (Gesellschaft zur Förderung der Qualitätssicherung medizinischer Laboratorien e. V.) in collaboration with the National Consultant Laboratory for Coronaviruses of the Institute of Virology of the Charité hospital in Berlin. In total, more than 1000 medical laboratories took part – among them laboratories of PTB, NIST (USA), and LGCNML (UK).

With the measurement procedures developed by these three national metrology institutes, it was, for the first time, possible to assign reliable and accurate target values to the control specimens sent that contained various RNA concentrations. A new and innovative technique was used for this purpose: droplet digital PCR (ddPCR). Hereby, the sample to be analyzed is divided into up to 20 000 droplets by means of an oil-inwater emulsion. In each of these droplets, the corresponding genomic sequence is amplified and analyzed. In this way, even very small amounts of viral RNA can be reliably detected and quantified.

By developing a ddPCR reference measurement procedure, PTB is significantly contributing to ensuring the quality and comparability of the measurements results and to assessing the test methods used.


Rainer Macdonald
Department 8.3
Biomedical Optics
Phone: +49 30 3481-7542
Opens local program for sending emailrainer.macdonald(at)ptb.de