Which AECOPD patients are likely to test positive for flu?

In patients with acute exacerbation of chronic obstructive pulmonary disease (AECOPD), higher body temperature and heart rate, along with low white cell counts, are strong predictors of testing positive for influenza, according to a recent Singapore study.

“Despite the potential benefits, indiscriminate influenza polymerase chain reaction (PCR) testing of all patients presenting with AECOPD is of questionable yield and may not be cost-effective,” the researchers said. “Given the uncertainty of influenza PCR testing in AECOPD, we performed this study with the aim of identifying predictors of influenza PCR positivity in AECOPD patients.”

From January 2016 to June 2017, 925 AECOPD patients (mean age 75 years, 87.9 percent men) underwent PCR testing for influenza. All patients contributed either nasal or throat swabs collected within 24 hours of admission. Clinical parameters and results of other medical investigations, all within the first day after admission, were obtained from electronic records.

Ninety patients tested positive for influenza, yielding an overall rate of 9.7 percent; sixty-eight had influenza A, while 22 had influenza B. PCR-positive patients had comparable baseline characteristics, comorbidities, COPD severity, blood pressure, oxygen status, X-ray findings, and blood biomarker profiles as counterparts without the infection. [Int J Chron Obstruct Pulmon Dis 2022;17:25-32]

On the other hand, body temperature was significantly elevated in PCR-positive patients (37.8°C vs 37.0°C; p<0.001), as was heart rate (107 vs 101 bpm; p=0.004). White cell count, in contrast, was significantly suppressed (9.45 vs 10.6 × 10⁹/L; p=0.007).

Moreover, significantly fewer PCR-positive patients demonstrated eosinophilic exacerbation than their infection-negative comparators (12.2 percent vs 28.7 percent; p<0.001).

Multivariate binary logistic regression analysis confirmed that heart rate (odds ratio [OR], 1.017, 95 percent confidence interval [CI], 1.004–1.030; p=0.011) and body temperature (OR, 1.324, 95 percent CI, 1.009–1.737; p=0.043) were significant and independent positive correlates of PCR positivity, while eosinophilic exacerbation (OR, 0.390, 95 percent CI, 0.202–0.756; p=0.005) was inversely so.

Grouping patients into quartiles of white cell count likewise confirmed an inverse association with PCR positivity. Those in the lowest quartile, for instance, were more than three times as likely to test positive for influenza than patients in the highest category (OR, 3.330, 95 percent CI, 1.690–6.562; p=0.001).

The study has important limitations that are worth noting. “Despite being one of the largest studies specifically assessing influenza PCR positivity in AECOPD patients, our single centre study in a predominantly Asian population may not be generalizable to other patient populations,” the researchers said, pointing specifically to the lack of a defined influenza season due to Singapore’s tropical climate.

Other notable methodological weaknesses included its retrospective nature, lack of comprehensive data on vaccination status, and the inability to control for other confounders, such as the time between disease onset and hospital presentation.

Despite such limitations, the present study was able to determine clinical factors that could help identify AECOPD patients at risk of influenza. “Although several of these factors may be seen in general respiratory viral infections, our study is one of the few in the literature to establish an independent relationship between them and influenza specifically,” the researchers said.

“These predictors, especially in combination, may have a role in guiding the clinician in identifying AECOPD patients in whom influenza PCR testing is appropriate,” they added.