THREAD .. It's so difficult trying to keep up with changes in case definitions. Based on the experience with SARS, two classifications helped greatly: "suspected case" and "probable case" who.int/csr/sars/cased…
For SARS, a "suspected case" was defined by WHO as a patient with (1) fever plus (2) cough or breathing difficulty plus (3) epidemiological link to other SARS cases or residing in an area with local transmission who.int/csr/sars/cased…
For SARS, a "probable case" was defined by WHO as a suspected case plus one of these additional criteria: (1) x-ray evidence of pneumonia or respiratory distress syndrome (RDS); (2) laboratory detection of SARS virus; (3) autopsy evidence of pneumonia/RDS who.int/csr/sars/cased…
For SARS, the category "confirmed case" or "laboratory-confirmed case" was not recommended by WHO because of a lack of available laboratory tests in 2003. who.int/csr/sars/cased…
If laboratory tests had been more widely available for SARS, a "confirmed case" would have been a "suspected case" or "probable case" with laboratory confirmation of infection, but the "probable cases" would still have been the most useful to track over time
The definition for "suspected case" can lack sensitivity -- influenza or other viral infections could also explain symptoms -- but it is valuable for identifying cases who should be isolated and tested
The definition for "probable case" should be able to pick up all serious infections and provide a more stable numerator for tracking the course of the epidemic - for SARS it was particularly useful because infections were severe enough to cause pneumonia or RDS
for SARS-CoV-2 we know many infections cause milder illness and some might even be asymptomatic, which complicates the case definitions
A few days ago, "clinically confirmed" cases in Hubei province were grouped together with "laboratory confirmed" cases -- a very sensible decision considering limited availability of laboratory tests time.com/5783401/covid1…
Today, the National Health Commission announce the latest case definitions to be used across China, returning to the requirement that "confirmed cases" are confirmed by laboratory testing and not only by clinical diagnosis nhc.gov.cn/yzygj/s7653p/2… (in Chinese)
Today's release is the sixth edition of the case definition. The first edition was issued on 15 January, and then updated on 18 Jan, 22 Jan, 27 Jan, and 4 Feb. Here is our translation of the current case definitions from the 6th edition (dated 18 February):
A "suspected case" (疑似病例) is a person who fulfils at least three of the four criteria listed below.
(1) an epidemiological link to Wuhan or its surroundings within 14d before illness onset, or contact with other confirmed cases within 14d before illness onset, or contact with another person who had fever or respiratory symptoms and ...
... is from Wuhan or its surroundings within 14d before illness onset, or contact with another person who had fever or respiratory symptoms from other communities with lab-confirmed cases within 14d before illness onset, or is part of a cluster of cases (聚集性发病).
(2) fever and/or respiratory symptoms. (3) radiographic findings of pneumonia. (4) normal or reduced white blood cell count soon after symptom onset, and/or(?*) reduced lymphocyte count soon after symptom onset.
*(We can't tell whether criteria #4 requires one or both conditions - in the 5th version it was clearly either/or)
A case could be "suspected" if they meet the epi criteria and two clinical criteria, or otherwise they would need to meet all three clinical criteria.
A "confirmed case" (确诊病例) is a suspected case who also has a positive test for SARS-CoV-2 by real-time RT-PCR, OR has a viral genetic sequence which is highly homologous to SARS-CoV-2.
A separate section about laboratory diagnosis procedures elaborates the specimen types in which SARS-CoV-2 virus nucleic acid has been identified: nasopharyngeal swab, sputum, other lower respiratory specimens, blood, stool.
In comparison, the case definitions issued by WHO on 31 January include definitions of "suspected", "probable" and "confirmed" cases who.int/publications-d…
WHO have two options for a "suspected case". First is a patient with severe acute respiratory infection (fever, cough, requiring hospitalization) AND no other etiology that explains the clinical presentation AND history of travel to China or residence in China in 14d before onset
Second is a patient with any acute respiratory illness AND either contact with a confirmed or probable COVID-19 case or worked in a healthcare facility where confirmed or probable COVID-19 cases were being treated.
WHO define a "probable case" as a suspected case with laboratory testing results reported by the laboratory as "inconclusive", or a positive result on a pan-coronavirus assay and without evidence of other respiratory pathogens.
WHO define a "confirmed case" as a person with laboratory confirmation of SARS-CoV-2, irrespective of clinical signs and symptoms. who.int/publications-d…
Where next? Surveillance is easier with consistent case definitions. We increasingly realise that many infections are mild, and it is valuable to distinguish the milder confirmed cases with those that require hospitalisation (i.e. not including those hospitalised for isolation)
I wonder if we should distinguish the suspected/probable/confirmed cases by severity, but that could lead to even more confusion ...
