Last year we wrote about our partnership with Beckman Coulter and the great work they are doing in the field of sepsis diagnostics. We are happy to announce that Beckman Coulter recently received FDA 510(k) clearance for their novel monocyte-based sepsis test called Early Sepsis Indicator, or ESId. ESId was approved as a biomarker used in the identification of patients with sepsis or at risk for developing sepsis in the emergency department and is currently the only hematologic parameter approved for this indication in the US. The ESId result is available on the Beckman Coulter DxH 900 hematology analyzer, the company’s latest high-volume analyzer, and is reported alongside routine CBC differentials as an optional add-on feature.

In our previous post on this topic, we briefly discussed ESId and the science behind it, but we would like to revisit the topic in more detail, and briefly cover the results of the pivotal trial on which its approval was based. For more information, please refer to the Diagnosis chapter of our Sepsis Clinical Guide app, under the Laboratory Tests section, Leukocyte Morphometric analysis, and to Beckman Coulter’s Early Sepsis Detection page.

What is the Early Sepsis Indicator

ESId is a novel hematologic parameter that measures abnormal deviations in the size of monocytes that occur during infections, and in particular during severe infections like sepsis. More specifically, ESId is a measure of the monocyte distribution width (MDW), a cellular morphometric indicator of cellular size variability similar to the reticulocyte distribution width (RDW) commonly used in diagnosing anemias and other conditions. MDW abnormalities are detected using Beckman Coulter’s Volume, Conductivity and Scatter (VCS) technology combined with automated statistical analysis of cellular morphometric characteristics that detects deviations from normal.

Monocytes are not the only leukocytes that undergo morphologic changes in sepsis, these also being seen in other leukocytes such as neutrophils. As is well known, infections cause the appearance of immature neutrophils known as band cells which have a different size and shape from their mature counterparts.(1) These cells are typically identified by their unilobular bean-shaped nucleus vs the multilobular nucleus of mature neutrophils. A high number of band cells (>10% of total WBC count) is one of the criteria of the systemic inflammatory response syndrome (SIRS) which is widely used in sepsis diagnosis. The morphologic and volumetric changes in the monocytes of sepsis patients, however,  are not due to the release of immature forms, which does not occur under normal circumstances, but primarily due to monocyte activation, differentiation, or a combination of both. More on this below.

So why use MDW to identify sepsis if we already have band cells? One reason is that band cells are not particularly specific for sepsis as they may appear in general inflammation, neoplasia, intoxication, and other conditions. The other reason is that band cells require manual counting, a laborious and imprecise process with a high margin of error.(2) MDW, on the other hand, can be determined automatically using modern hematology analyzing systems like VCS, and has been identified in a number of studies as being a good biomarker for distinguishing sepsis from other more benign infections, especially when combined with abnormal white blood cell (WBC) counts.(3, 4)

ESId Pivotal Trial Results

In the ESId pivotal trial, an abnormal WBC count plus MDW performed very well in its ability to distinguish sepsis from more benign infections or SIRS, with an AUC of 0.85 (95% CI 0.83-0.88) which was significantly superior to abnormal WBC alone (p<0.05). An abnormal MDW strengthened a clinician’s suspicion of sepsis by 42% vs an abnormal WBC count alone, and by 99% when combined with abnormal clinical signs and symptoms (high or low temperature, and elevated heart and respiratory rates). Conversely, a normal MDW improved clinician confidence in ruling out sepsis by 63% vs a normal WBC count alone, and by 96% if clinical symptoms were also considered (normal temperature, heart, and respiratory rates). In the studied population, which had a pre-test probability (prevalence) of 17.8%, if all SIRS criteria and MDW were positive, the post-test probability of sepsis was 89.9%, whereas if all SIRS criteria were negative and MDW was normal, the post-test probability of sepsis was 0.3%.(5)

Monocytes – Complex Sentinel Cells

It is interesting to explore why monocytes make good targets for sepsis biomarkers. As we might remember from basic immunology, monocytes are one of the sentinel cells of the innate immune system, the part of the immune system that responds first in a non-specific way to infectious insults (as opposed to the specific response of the adaptive immune system). The primary role of monocytes is to be the early detectors and alarm sounders of infection in the body. About half the monocytes released from the bone marrow circulate through the bloodstream (circulating pool), while half attach to the endovascular wall (marginated pool), a number of which migrate into tissues through diapedesis. In the tissues, monocytes can differentiate into macrophages, whose primary function is phagocytosis of bacteria and foreign organisms and debris, and dendritic cells, which have antigen presenting properties and serve as a bridge between the innate and adaptive immune systems. The marginated pool can also be a reservoir of rapid monocyte recruitment during inflammation and infection.(6)

Monocytes express on their surface an important protein called CD14 which is a receptor for bacterial wall components, specifically, the lipopolysaccharides (LPS/endotoxins) found in Gram-negative bacteria, and LAM and HSP-60 components found in Gram-positive bacteria.(7) Binding of these bacterial components to CD14 triggers bacterial phagocytosis and the release of a host of proinflammatory cytokines like TNF-alpha and IL-1 which act as signals for immune system activation.(8) Monocyte activation also causes the release of the soluble form of CD14, sCD14, which also binds LPS, and has additional immunomodulatory and monocyte activating effects.(9)

Monocytes are generally classified into three subtypes according to the level of expression of the CD14 receptor and another receptor protein called CD16 which is an antibody FcγIII receptor involved in antibody-mediated cellular cytotoxicity. Classical monocytes (CD14++CD16-) are primarily phagocytic in nature, non-classical monocytes (CD14+CD16++) are more inflammatory in nature and have antigen presenting properties, and intermediate monocytes (CD14++CD16+) fall somewhere in the middle.(10) The large majority (appx 85%) of circulating monocytes are classical, while the remainder (appx 15%) are intermediate and non-classical.(11) Monocytes are released from the bone marrow in their classical form, and under normal circumstances, the vast majority (99%) of these leave the circulation (either through apoptosis or migration to tissues), while a small portion (1%) convert to intermediate monocytes and later to non-classical monocytes.(11) Infections cause profound shifts in monocyte numbers and populations, (10, 1112) which combined with differences in cellular size between monocyte subtypes (1013, 14) may partially explain the increased MDW seen in sepsis.

Clearly, the monocyte is a key component of the early immune response that occurs in infection and sepsis and an important source of potential sepsis biomarkers. Monocyte activation and differentiation cause an increase in size variability that is detected as an increase in MDW by hematology analyzers. Soluble sCD14 is also being explored as a sepsis biomarker, which in this context is referred to as presepsin.(15) Future sepsis tests may use monocyte immunocytochemical properties to more accurately detect monocyte subpopulation shifts that occur in sepsis.

Most importantly, because monocytes are an early immune system responder, they may serve as earlier markers of sepsis or impending sepsis than other biomarkers. In a recent study by Mammen et al, VCS parameters, including the mean neutrophil and monocyte volumes, of patients who developed overt culture-positive sepsis within 48 to 72 hours of testing, were similar to the parameters of patients with confirmed sepsis at the time of testing, suggesting that morphometric changes in these cells may precede the frank onset of sepsis.(16) However, more studies remain to be done to determine the exact timing of changes in MDW and other leukocyte morphometric biomarkers in the course of sepsis development. Ideally, a sepsis biomarker should tell us sepsis is coming before it actually arrives.


In conclusion, we believe that MDW, and in particular ESId as a validated MDW parameter, is a valuable addition to sepsis detection tests that, in combination with WBC and clinical symptom assessment, can help expedite and solidify the diagnosis of sepsis in emergency room patients, where sepsis most often first presents. Its availability as part of a routine WBC differential is also a plus as this means results will turn around very quickly and not involve additional specialized equipment. For more information on ESId and the Dx900 hematology analyzer, please visit Beckman Coulter’s Early Sepsis Detection page.

We hope this has been a helpful review of MDW and monocyte physiology. We will be keeping an eye on ESId and other monocyte-based sepsis biomarkers and report further developments in this area in the future.

Daniel Nichita, MD



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