*2.3. Temperature Correlation*

Distal site temperatures averaged 119.95 ◦F after a 1-min flush. Distal site temperature was not statistically related to the distal site *Legionella* positivity (*p* = 0.170). Distal sites with no *Legionella* recovered trended towards higher hot water return temperatures, with an average 9.7 ◦F higher temperature. However, this was not statistically significant (*p* = 0.0687).

When using 124 ◦F, the minimum recommended return circulation temperature [24], as a threshold for *Legionella* distal site positivity, there was an association between the two values (*p* = 0.013) (Figure 4). This hot water return temperature threshold value had a sensitivity of 65% (13/20) and a specificity of 72% (18/25) for determining the *Legionella* distal site positivity. However, the positive predictive value was only 65% (13/20) and the negative predictive value was 72% (18/25). This recommended temperature threshold is a poor screening test for distal site positivity (receiver operating characteristic (ROC) curve area = 0.68). In the buildings with a hot water return temperature > 124 ◦F, 28% (7/25) were still positive for *Legionella* in distal sites.

**Figure 4.** *Legionella* distal site positivity was seen in systems with hot water return line temperatures above and below the recommended threshold. The red vertical line represents the 124 ◦F threshold value.

#### *2.4. Incoming Cold-Water Chlorine Concentration*

Incoming cold-water chlorine concentration was analyzed to determine if the concentration could be used as a predictor of *Legionella* distal site positivity. Incoming cold-water chlorine measurements were available from 20 buildings. Using 0.5 mg/<sup>L</sup> as the threshold for the acceptable level of free residual chlorine found in drinking water [25], there was no correlation between incoming chlorine concentration and *Legionella* positivity (*p* = 0.582) (Figure 5). This 0.5 mg/<sup>L</sup> incoming chlorine threshold fails as a screening tool for *Legionella* distal site positivity (ROC curve area = 0.47).

**Figure 5.** *Legionella* distal site positivity was seen in buildings with incoming cold-water free chlorine above and below the recommended threshold. The red vertical line represents the 0.5 mg/<sup>L</sup> threshold value.
