In an ECDC report from Jan 17, it is said that
In China, 763 close contacts have been identified and monitored. Of these, 644 have completed the observation period, while 119 remain under medical observation. So far, none has tested positive for 2019-nCoV [7,10].
This seems a (very) striking result. With further time (and hopefully investigations) in hindsight, is there anything that explains why all those early tests of close contacts (of known cases) were themselves negative? Were the tests faulty in some way?
According to some much more recent media reports, Chinese tests kits sent to Europe or even Turkey have been found faulty, although the percentage is not made clear in most cases, except [oddly] for Turkey:
In Turkey, which imported its first batch of “several thousand” kits in early March, officials determined an accuracy rate of less than 35%, according to a senior official with direct knowledge of the matter. Their use was immediately suspended and new tests sourced from a different Chinese supplier. They arrived last week and had an accuracy rate of about 90%, according to the Turkish official.
(The article date is 1 April, but I don't think it's a hoax.) I don't know if this can possibly be related to the earlier Chinese non-detections or not... but also interestingly that low (~30% accuracy number) has been mentioned before:
In February, Wang Chen, a director at the state-run Chinese Academy of Medical Sciences, estimated that the nucleic acid tests used in China were accurate at identifying positive cases of the coronavirus only 30%-50% of the time.