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[1226] Use of the Mean Neutrophil Volume Greatly Improves the Laboratory Diagnosis of Bacteremia

S Arya, K Quillen, S Daniels, F Chaves. Boston Univ Medical Center, Boston, MA

Background: In previous studies (USCAP 2005, Abstract 1205), we have demonstrated that the cell volume of neutrophils (Mean Neutrophil Volume, MNV) increases during acute bacterial infection, and that the sensitivity of this new parameter is superior to that of other commonly used tests, such as the WBC and the neutrophil percentage (%Neu). However, these case-control, proof of concept studies were not meant to fully evaluate the diagnostic potential of the MNV. Since the positive and negative predictive values (PPV / NPV) depend on disease prevalence they could not be calculated. This follow up study was designed to better evaluate the diagnostic value of the MNV, both alone and in combination with other markers for infection, in cases with a high clinical suspicion for this condition.
Design: The study population included all emergency room patients who had a blood culture (BC) ordered on 10 random non-consecutive days (n=150). Data collected included WBC, Absolute Neutrophil Count (Abs Neut), % Neu, and the MNV, tested on blood samples obtained up to 2 days prior to the BC collection. The sensitivity, specificity, PPV and NPV for predicting a positive BC were calculated for each of these parameters. BCs that yielded bacteria likely to be contaminants, such as coagulase negative Staphylococci, were considered negative.
Results: Twelve BCs were reported positive. The value of each of the parameters studied in predicting a positive BC is shown in Fig 1a. Although the MNV showed a lower sensitivity then in previous studies, it still had the best PPV. All parameters had sub-optimal PPVs, reflecting the difficulty of confirming a diagnosis of bacteremia. When we analyzed the MNV and %Neu in combination, we obtained a significant improvement in the PPV, while maintaining the same NPV and Specificity (Fig1b).
Conclusions: The MNV could greatly improve the value of laboratory testing in the diagnosis of bacteremia. It has a better diagnostic performance than other tests traditionally used for this purpose. It is a morphological parameter obtained automatically, at no extra cost, during a regular CBC with differential, and could be most useful if combined with quantitative parameters such as the %Neu.


Category: Infections

Tuesday, March 27, 2007

Poster # 178, Tuesday Morning