Volume, conductivity, and scatter parameters of leukocytes as early markers of sepsis and treatment response

Introduction

Sepsis is a major cause of morbidity and mortality in hospitalized patients worldwide, especially in a developing country like India. The early diagnosis of sepsis is extremely important to reduce the high mortality by prompt initiation of antibiotics. Blood culture remains the gold standard for diagnosing bacteremia or sepsis.[1] Although viral and fungal agents are also implicated in sepsis, bacteria is still the most important implicated agent. Complete hemogram along with differential leukocyte count are the first parameters to suggest infections.[2] Increased total leukocyte count (TLC), immature/total neutrophil ratio, and neutrophilia are associated findings in sepsis. The characteristic morphological changes in the size of the cell, the density of the nucleus, nuclear lobes, along with the presence of toxic granules, vacuolization, and occasional Döhle bodies, are characteristically seen in sepsis.[34] The identification of these characteristics on peripheral smear is a subjective finding, the identification of which depends on the expertise in microscopy. Blood culture too has limitations, such as it has low-sensitivity, takes time for growth, and is easily amenable to contamination. Moreover, a positive blood culture report takes at least 48 h, which may delay the initiation of early antibiotic therapy by the treating physician.[1]

The newer hematology analyzers have completely revolutionized the hemogram analysis. The Coulter LH 750 hematology analyzer (Beckman Coulter, Fullerton, CA, USA) obtains data from >8000 white blood cells (WBCs) in a few seconds, using impedance to measure the cell volume (V) for the size of all cells, radio-frequency opacity to characterize the conductivity (C) for the internal composition of each cell, and a laser beam to measure the light scatter (S) for cytoplasmic granularity and nuclear structure. This volume, conductivity, and scatter (VCS) technology, thus helps in quantifying not only the number of cells but also the internal characteristics of a cell which can be compared with the morphological appearance of WBCs on peripheral smear examination.[567]

This VCS feature can be used as a tool for the early diagnosis of sepsis.[78910] The usefulness of VCS parameters can be assessed by their changes at the time of sepsis. Although there have been previous studies which support the same, their clinical relevance can be judged by the change in parameters with the course of antibiotic treatment.[91112] Therefore, in the present study, we have tried to investigate the significance of VCS parameters such as mean neutrophil volume (MNV), conductivity (MNC), and scatter (MNS), mean monocyte volume (MMV), conductivity (MMC), and scatter (MMS) in diagnosing bacteremia and correlate them to antibiotic course. We have also tried to find a significant cutoff value of MNV and MMV that could aid to be supportive evidence in diagnosing bacteremia or sepsis.

Methods

This retrospective observational study was conducted at a tertiary care hospital in North India over a period of 6 months.

Results

Data from 134 patients with a positive bacterial blood culture and 100 controls were analyzed.

Volume, conductivity, and scatter parameters

Neutrophils

A significantly higher MNV was seen in sepsis patients than that of the controls (165.43 ± 18.21 vs. 140.59 ± 7.6, P = 0.001). MNC (128.9 ± 7.8 vs. 155.36 ± 4.27, P = 0.001), and MNS (138.58 ± 9.7 vs. 142.26 ± 6.7, P = 0.027) were significantly lower in cases as compared to that of the controls.

Monocytes

Similarly, monocytes showed a significantly higher mean volume (MMV) in the sepsis patients than that of the controls (179.8 ± 14.16 vs. 164.54 ± 9.6, P = 0.001). MMC (110.8 ± 6.3 vs 130.6 ± 2.9, P = 0.001) was found to be lower in cases when compared with the controls.

Changes in white blood cell and volume, conductivity, and scatter parameters with treatment

In 134 blood culture-proven cases of sepsis, the changes in WBC and VCS parameters were evaluated on successive measurements on day 3 and day 7 after the antibiotic treatment was initiated [Table 2]. There was statistically significant decrease in MNV and MMV (P < 0.001 for both) values with antibiotic treatment, although NE% (P = 0.72) and MO% (P = 0.33) did not decrease significantly. MNC, MMC, and MMS showed increased values with treatment which was statistically significant (P < 0.01).

Table 2 Changes in white blood cell and volume, conductivity, and scatter parameters with treatment (n=134)

	Mean/median* value			P
	Day 0	Day 3	Day 7	Overall	Day (0 vs. 3)	Day (0 vs. 7)	Day (3 vs. 7)
Neutrophils (%)	75.31±15.35	74.86±14.18	74.15±16.63	0.7	0.77	0.40	0.62
MNV	166.20±15.79	158.20±15.49	155.04±14.47	<0.001	<0.001	<0.001	0.001
MNC	127.57±7.51	130.85±6.65	129.38±7.14	<0.001	<0.01	0.12	0.14
MNS	137.35±11.62	139.70±7.35	140.03±9.30	0.03	0.06	0.03	0.70
Monocytes (%)	7.8*(0.4-33.6)	7.3*(0.6-25.2)	6.8*(0.4-28.8)	0.33	0.50	0.03	0.10
MMV	182.01±15.58	178.07±11.65	176.28±10.63	<0.001	0.001	<0.001	0.012
MMC	110.56±6.13	111.57±7.39	112.94±8.30	0.003	0.12	0.002	0.04
MMS	90.18±5.22	92.16±4.62	93.09±3.73	<0.001	<0.001	<0.001	0.001

Values are expressed as mean±SD for parametric or median* (minimum-maximum) for non-parametric results. SD = Standard deviation, MCV = Mean corpuscular volume, TLC = Total leukocyte count, MNV = Mean neutrophil volume, MNC = Mean neutrophil conductivity, MNS = Mean neutrophil scatter, MMV = Mean monocyte volume, MMC = Mean monocyte conductivity, MMS = Mean monocyte scatter

Evaluation of sensitivity, specificity of volume, conductivity, and scatter parameters for predicting sepsis

Sensitivity and specificity for TLC, percentage neutrophils, monocytes, MNV, and MMV were calculated at designated cutoff points that were taken from the ROC curves [Table 3 and Figure 1]. A cutoff value of 150.2 for MNV yielded a high sensitivity and specificity of 79.1% and 95%, respectively, with an area under the curve (AUC) of 92.3%. With a cutoff of 168.3, MMV had a sensitivity of 80.6% and specificity of 77.5% with an AUC of 82.97%.

Table 3 Determination of cutoffs, sensitivity, specificity, and area under curve for mean neutrophil volume and mean monocyte volume

	Differential groups	Cutoff points	Sensitivity	Specificity	Area under curve (%)	95% CI
						Lower bound	Upper bound
Controls versus cases	TLC (×103/μL)	10.05	57.5	87.5	68.1	60.5	75.6
	Neutrophil (%)	60.05	79.9	57.5	79.8	73.2	86.4
	Monocyte (%)	7.15	51.5	32.5	48.3	39.8	56.8
	MNV	≥150.2	79.1	95	92.3	88.3	96.3
	MMV	≥168.3	80.60	77.50	83.0	76.0	89.9

CI = Confidence interval, TLC = Total leukocyte count, MNV = Mean neutrophil volume, MMV = Mean monocyte volume

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Figure 1

Receiver-operator characteristic curves for various neutrophil and monocyte parameters to differentiate the cases of sepsis from the healthy controls

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Discussion

This retrospective observational study was conducted to evaluate the morphological changes of the WBCs obtained from the automated analyzer and its importance in predicting septicemia. LH 750 hematology analyzer examines the large numbers of cells to provide the reliable information about the number, size, nuclear complexity, and granularity of the WBCs. These parameters can be measured by the VCS technology. Such information has definitive implications in a clinical setting.[1314] In this study, we evaluated the VCS parameters of 134 septic patients with positive blood cultures and their response to treatment, comparing them to 100 healthy controls.

It is a known fact that septicemia is characterized by the appearance of immature neutrophils such as bilobed neutrophils, bands, metamyelocytes, and myelocytes in the peripheral blood.[34] Morphologically, these immature neutrophils are larger and have a relatively simple nucleus. Such changes are reflected by raised MNV and decreased MNC and MNS in these cases compared to the controls.[1112] This study also reported such a change in pattern in monocytes which shows their role in response to the infections.

This study shows a retrospective analysis of the VCS parameters on the day of diagnosing septicemia and to find the effect of treatment, on day 3 and day 7. It was seen that the NE% showed no statistical change [Table 2], (P = 0.7) with the treatment, which shows that it cannot be used to judge the response of treatment from the laboratory point of view. At the same time, it was observed that there was statistically significant decrease in size of the neutrophils and monocytes confirmed by the normalization of MNV and MMV [Table 2] (P < 0.001 for both). The finding is further supported by a significant increase in MNC and MNS (though not significant), which reflects the normalization of WBCs in the peripheral blood causing immature neutrophils to be removed by apoptosis and being replaced by mature ones, thus increasing granularity and nuclear complexity with the treatment.

We further evaluated that whether a cutoff value can be determined which can be used in clinical practice to identify the cases of septicemia on the initial hemogram run on automated analyzers. ROC curve was computed to compare the sensitivity and specificity of the MNV and MMV. It was observed that taking a cutoff of ≥150.1, MNV gave a sensitivity of 79.85% and a specificity of 95% with an AUC of 92.3. The value is slightly different from studies from other parts of the world, which suggests differences in the study population and might reflect in their VCS parameters also.[7111214] It is, therefore, important to establish normal ranges of these VCS parameters in one's own population before interpreting them in clinical significance.

Other investigators have also reported MNV as having higher sensitivity for predicting infection than TLC and the percentage of neutrophils. If the diagnosis of infection had been based only on the WBC count or the percentage of neutrophils, the correct diagnosis easily could have been overlooked. Therefore, we believe that the MNV has the potential to be used as an additional indicator of acute bacterial infection, particularly when the other hematologic parameters do not correlate well with the patient's clinical signs and symptoms.[78131516]

WBC counts seem to be an insensitive discriminator for the systemic infections[1718] because sepsis can be associated with normal or decreased leukocytes caused by an increased turnover and/or toxic suppression of bone marrow maturation.[12] Clinical scores with regard to organ failure and outcome such as the Acute Physiology and Chronic Health score should also be used to quantitatively evaluate the patient group. Other markers that can be used in similar settings include C-reactive protein and procalcitonin.[1920] However, they are relatively expensive. A combination of markers can also be used to increase the diagnostic yield.[2122]

Several other clinical applications of the VCS technology remain to be studied. With the vast amount of data that could be obtained relating to the morphologic features of neutrophils, lymphocytes, monocytes, and eosinophils separately, the prospects of their clinical use are promising. Other possible clinical applications could include the early diagnosis of other infections like viral infections that are known to change the morphologic features of lymphocytes.[23]

Conclusion

VCS parameters can be an important diagnostic tool that can serve as an adjunct to the clinical diagnosis of bacterial sepsis and can help in the early initiation of empirical antibiotic therapy.