Q&A column

Editor: Frederick L. Kiechle, MD, PhD

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Q. Can bronchoalveolar lavage specimens from multiple lobes be pooled for culture? Can multiple biopsies from the same joint be pooled for culture?

A.June 2022—These are straightforward questions with somewhat complex answers. In general, the advantages of pooling samples include saving time, labor, raw materials, laboratory space, and money. Yet there are downsides. One potential disadvantage is the dilution of microbes within a specimen to below the limit of detection or threshold of clinical significance. Another potential drawback is the loss of sampling-site information that could be used for clinical-pathologic or radiologic-pathologic correlation.

With these factors in mind, it is not uncommon for laboratories to pool samples for fungal or mycobacterial cultures, including from bronchoalveolar lavage (BAL) and joint tissue biopsies. However, there is a lack of literature that examines this practice. For other types of organisms, a more nuanced discussion is needed, and practices may vary across laboratories. Additional considerations for pooling also differ between BAL specimens and joint tissue biopsies.

For routine microbiological analysis of BAL fluid, literature and guidelines predominantly center around the diagnosis of hospital-acquired pneumonia (HAP) and ventilator-associated pneumonia (VAP). Infectious Diseases Society of America (IDSA) guidelines describe the use of quantitative BAL cultures for diagnosing HAP/VAP, employing a cutoff of 10⁴ CFU/mL for clinical significance, though the guidelines favor noninvasive methods, such as endotracheal aspiration with semiquantitative culture.¹

The guidelines do not address the practice of pooling samples from different bronchial segments, but there are a handful of papers that indirectly address this practice. Zaccard, et al., calculated that pooling samples from the right and left lung would have allowed detection of all bacterial species originally above the 10⁴ CFU/mL cutoff in 92.5 percent (160/173) of samples in their study.² In 3.5 percent (6/173) of samples, at least one, but not all, bacterial species would have been detected. Four percent (7/173) of the samples would have been falsely negative. As would be expected, failure to detect bacterial species occurred with lower concentrations of bacteria near the 10⁴ CFU/mL cutoff in the nonpooled samples. A study by Jonker, et al., found 74 percent discordance between radiologic findings and sites of BAL culture positivity, and there is disagreement in the literature about whether the presence or absence of infiltrates on radiography correlates with culture positivity at all.^3-5 These and other studies have been largely conducted in surgical and trauma patients. The methodologies for BAL fluid collection vary. Depending on the methods used, dilution due to pooling may be comparable to dilutions observed in higher volume BAL procedures.^6,7 Ultimately, laboratories that want to pool BAL samples might first discuss this undertaking with relevant stakeholders or conduct an internal study to evaluate the effects of pooling on culture quantitation and sensitivity, or both.