Blood neutrophil counts in HIV-infected patients with pulmonary tuberculosis: association with sputum mycobacterial load

Abstract

Background: Increasing evidence suggests that neutrophils play a role in the host response to Mycobacterium tuberculosis. We determined whether neutrophil counts in peripheral blood are associated with tuberculosis (TB) and with mycobacterial load in sputum in HIV-infected patients.

Methodology/principal findings: Adults enrolling in an antiretroviral treatment (ART) clinic in a Cape Town township were screened for TB regardless of symptoms. Paired sputum samples were examined using liquid culture, fluorescence microscopy, and the Xpert MTB/RIF assay. Absolute neutrophil counts (ANC) were measured in blood samples. Of 602 HIV-infected patients screened, 523 produced one or more sputum samples and had complete results available for analysis. Among these 523 patients, the median CD4 count was 169×10(9)/L (IQR, 96-232) and median ANC was 2.6×10(9)/L (IQR, 1.9-3.6). Culture-positive pulmonary tuberculosis was diagnosed in 89 patients. Patients with TB had a median ANC of 3.4×10(9)/L (IQR, 2.4-5.1) compared to 2.5×10(9)/L (IQR, 1.8-3.4) among those who were culture negative (p<0.0001). In multivariable analyses, having pulmonary TB was associated with an adjusted risk ratio (aRR) of 2.6 (95%CI, 1.5-4.5) for having an ANC level that exceeded the median value (ANC ≥2.6×10(9)/L; p = 0.0006) and an aRR of 6.8 (95%CI, 2.3-20.4) for having neutrophilia defined by a neutrophil count exceeding the upper limit of the normal range (ANC >7.5×10(9)/L; p = 0.0005). Patients were then classified into four mutually exclusive groups with increasing sputum mycobacterial load as defined by the results of culture, Xpert MTB/RIF and sputum smear microscopy. Multivariable analyses demonstrated that increasing sputum mycobacterial load was positively associated with blood ANC ≥2.6×10(9)/L and with neutrophilia.

Conclusions/significance: Increased blood neutrophil counts were independently associated with pulmonary TB and sputum mycobacterial burden in this HIV-infected patient group. This observation supports the growing body of literature regarding the potential role for neutrophils in the host response to TB.

Figure 1. Flow diagram showing patients included in the analysis.

Figure 2. Box and whisker diagram of absolute neutrophil count among those (a) with and without pulmonary TB and (b) and among those with increasing levels of sputum mycobacterial load.

The middle of the box represents the median value and the outer box edges represent the 25^th and 75^th percentiles (IQR). The ends of the whiskers represent the lower and upper adjacent values. Outliers as defined by any data point above the upper adjacent value, were excluded from both plots. The dashed line at ANC = 7.5×10⁹/L represents the upper limit of the normal ANC range, above which is considered neutrophilia.

Figure 3. The proportion (with 95% confidence intervals) of patients with HIV-associated TB who have neutrophilia, stratified by increasing level of sputum mycobacterial load.

Figure 4. Log adjusted risk ratios (aRR) for increased blood neutrophil counts stratified by low (n = 25), intermediate (n = 40) and high (n = 24) sputum mycobacterial load.