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How can we tell if an increase in cases is the result of an increase in testing or something else?

This article was published on
July 8, 2021

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A lot can be learned and based off of the percent-positive rate (e.g. how many tests result positive out of all the tests taken) and the number of cases in total. We cannot assume that an increase in cases or a growing percent-positive rate is purely a result of an increase in testing instead of a growing outbreak. Instead, we need to look at all of them together. A rise in the number of reported cases of COVID-19 could be related to an expansion of testing if the percentage of positive tests decreases or stays the same at the same time that the number of cases increases. Should percentage of positive tests increase while case counts also go up, this indicates that we cannot entirely blame the increase on expanded testing. The biggest indicator of a growing outbreak is if the percentage of positive tests increases along with the number of cases despite testing data staying the same or decreasing. When testing is not always widely available and reserved just for symptomatic people, the percent positivity will increase as with the number of cases. If testing is expanded and made more available, we will gain a better understanding of the true number of cases and percent-positive rate. If this percent positivity continues to grow along with the number of cases, this would be an indicator that the outbreak is worsening.

A lot can be learned and based off of the percent-positive rate (e.g. how many tests result positive out of all the tests taken) and the number of cases in total. We cannot assume that an increase in cases or a growing percent-positive rate is purely a result of an increase in testing instead of a growing outbreak. Instead, we need to look at all of them together. A rise in the number of reported cases of COVID-19 could be related to an expansion of testing if the percentage of positive tests decreases or stays the same at the same time that the number of cases increases. Should percentage of positive tests increase while case counts also go up, this indicates that we cannot entirely blame the increase on expanded testing. The biggest indicator of a growing outbreak is if the percentage of positive tests increases along with the number of cases despite testing data staying the same or decreasing. When testing is not always widely available and reserved just for symptomatic people, the percent positivity will increase as with the number of cases. If testing is expanded and made more available, we will gain a better understanding of the true number of cases and percent-positive rate. If this percent positivity continues to grow along with the number of cases, this would be an indicator that the outbreak is worsening.

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What our experts say

A lot can be learned and based off of the percent-positive rate (e.g. how many tests result positive out of all the tests taken) and the number of cases in total. We cannot assume that an increase in cases or a growing percent-positive rate is purely a result of an increase in testing instead of a growing outbreak. Instead, we need to look at all of them together. A rise in the number of reported cases of COVID-19 could be related to an expansion of testing if the percentage of positive tests decreases or stays the same at the same time that the number of cases increases. Should percentage of positive tests increase while case counts also go up, this indicates that we cannot entirely blame the increase on expanded testing. The biggest indicator of a growing outbreak is if the percentage of positive tests increases along with the number of cases despite testing data staying the same or decreasing. When testing is not always widely available and reserved just for symptomatic people, the percent positivity will increase as with the number of cases. If testing is expanded and made more available, we will gain a better understanding of the true number of cases and percent-positive rate. If this percent positivity continues to grow along with the number of cases, this would be an indicator that the outbreak is worsening.

Context and background

It is important to evaluate the number of cases, availability of testing, and percent positive rate when evaluating if a true climb in cases is due to a growing hotspot or just an increase in testing availability. Often, lack of testing availability can cloud this issue, which is why evaluating this piece of information in comparison to the growth in case counts while considering percent positive, provides a more accurate picture.

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