Table 2. Proportion of tests positive for severe acute respiratory syndrome coronavirus 2 with median Ct values in pooled testing and laboratorywide clinical testing datasets, subset by testing indication*.
| Dataset | No. positive samples/no. total samples (%) |
Median Ct value (IQR) |
|||||
|---|---|---|---|---|---|---|---|
| All | First | Follow-up | All | First | Follow-up | ||
| Pools of 8† | 58/880 (6.6) | 24/657 (3.7) | 34/223 (15.2) | 31.4 (22.1–35.5) | 24.4 (18.4–33.1) | 34.1 (29.0–36.8) | |
| Pools of 4‡ | 38/768 (4.9) | 28/491 (5.7) | 10/277 (3.6) | 29.3 (20.3–33.9) | 27.5 (19.4–32.6) | 32.2 (24.9–34.5) | |
| Hologic§ | 10,000/52,272 (19.1) | NA | NA | 26.2 (20.7–32.6) | NA | NA | |
| Laboratory-wide¶ | 1,358/74,162 (1.8) | 1,109/66,070 (1.7) | 249/8,092 (3.1) | 28.5 (23.0–34.3) | 27.2 (22.2–32.4) | 34.2 (29.0–37.4) | |
| March | 555/8,896 (6.2) | 489/8,557 (5.7) | 66/339 (19.5) | 26.7 (21.9–31.5) | 26.4 (21.8–31.2) | 28.6 (22.6–35.2) | |
| April | 518/22,671 (2.3) | 404/21,167 (1.9) | 114/1,504 (7.5) | 30.6 (24.8–36.0) | 28.8 (22.7–34.6) | 35.4 (32.9–38.0) | |
| May | 172/21,833 (0.8) | 136/19,505 (0.7) | 36/2,328 (1.5) | 27.5 (23.3–34.7) | 26.1 (22.5–31.3) | 35.4 (30.4–37.3) | |
| June | 113/20,762 (0.5) | 80/16,841 (0.5) | 33/3,921 (0.84) | 28.2 (21.2–33.6) | 27.4 (21.3–32.7) | 30.6 (20.2–34.4) | |
*Ct, cycle threshold; IQR, interquartile range; NA, not available.
†Pools of 8 specimens were tested in our clinical laboratory during June 10–19, 2020.
‡Pools of 4 specimens were tested in our clinical laboratory during July 6–23, 2020.
§Hologic dataset comprises specimens tested clinically by Panther Fusion (https://www.hologic.com) during March 1–July 31, 2020 at 2 different external sites. These data were used to perform in silico sensitivity analysis to evaluate pool sizes of 3 and 5.
¶Composed of clinical specimens obtained during March 1–June 24, 2020.