36. What is the optimal pleural fluid volume?

Abstract

<h3>Background</h3> Cytological examination of pleural fluid is a fast, efficient, and minimally invasive technique for establishing a diagnosis of cancer. Although many studies have looked at diagnostic yield comparing cytology with other modalities, few have evaluated the optimal pleural fluid volume needed to make a diagnosis of malignancy. <h3>Aims</h3> To determine if a diagnosis of malignancy becomes more likely as the volume of pleural fluid sampled increases. <h3>Method</h3> The volumes of cytologically diagnosed malignant pleural fluid samples submitted to The Prince Charles Hospital Cytology Laboratory between 2000 and 2009 were retrospectively compared to a control group comprising the negative samples submitted during the months of January of the same period. A total of 326 malignant samples from 271 patients were submitted during this interval. The control group consisted of 61 samples from 57 patients. Six groups of pleural fluid volume were establish-ed:<5mL, 5–10mL, >10–20mL, >20–50mL, >50–200mL, and >200mL. <h3>Results</h3> 27.6% of the malignant samples were in the >20–50mL group compared to 8.2% of the controls; 32.8% of the control samples were in the <5mL group compared to 12.6% of the malignant samples. The likelihood of a malignant diagnosis being reached increases as the volume of fluid increases. Conversely, as the volume approaches zero, the likelihood of returning a malignant diagnosis approaches zero. The peak in likelihood is at 20–200mL but odds remain high at >200mL. <h3>Conclusion</h3> Submitting 20–200 mL is optimal and <10mL is not advisable as it is likely to return a false negative result. Possible reasons for the decline at >200mL are discussed.