Evaluation of the delta neutrophil index from an automated blood cell analyser in septic dogs

Highlights

• The delta neutrophil index (DNI) is a potential marker for canine sepsis.

• The DNI appears to have value in predicting septic shock in dogs.

• The DNI is correlated with the presence of immature and toxic neutrophils evaluated on blood smears in dogs with sepsis.

Abstract

Immature granulocytes (IG) are a marker of severe inflammatory states in human beings and animals, and have been linked to a diagnosis of sepsis and poor prognosis. The delta neutrophil index (DNI), automatically calculated by a haematological analyser, provides an estimate of circulating IG. In particular, an increased DNI value has been associated with the severity of sepsis, and mortality, in critically ill human beings. The aims of this study were to determine the DNI reference interval (RI) in healthy dogs, and to evaluate its diagnostic and prognostic significance in dogs with sepsis. A total of 118 dogs with sepsis undergoing a complete blood cell count (CBC) at the time of hospital admission were included retrospectively. Dogs with sepsis were compared to 20 dogs with primary immune-mediated haemolytic anaemia (IMHA) and 99 healthy controls. The DNI RI was set from 0 to 9.2%. The DNI was significantly higher in dogs with sepsis compared to dogs with IMHA and healthy dogs (P < 0.001), and significantly higher in dogs with septic shock compared to septic dogs without circulatory failure (P < 0.03). No differences were detected between survivors (78/118) and non-survivors (40/118). Septic dogs with a DNI above the RI had significantly higher frequencies of IG and toxic neutrophil changes on manual blood smear evaluation (P = 0.03 and P < 0.001, respectively). The DNI had a fair performance in identifying dogs with sepsis in this population and predicted septic shock. Larger prospective studies are needed to validate DNI measurement in dogs and to test its clinical utility.

Introduction

Sepsis is a common disease recognised in the intensive care unit (ICU) and results in high morbidity and mortality in human and veterinary patients (Silverstein, 2012, Gotts and Matthay, 2016). Despite a reduction of in-hospital mortality in the last 40 years, the incidence of sepsis seems to have increased in human beings (Gotts and Matthay, 2016) and the early diagnosis of this syndrome remains a major goal in order to implement prompt and effective treatment (Silverstein, 2012, Hayden et al., 2016).

Neutrophil precursors, including band neutrophils, metamyelocytes and myelocytes, are defined as immature granulocytes (IGs) (Stockham and Scott, 2008, Mare et al., 2015). They are released into the circulation in response to severe inflammation and are included in the diagnostic criteria for systemic inflammatory response syndrome (SIRS) in human beings and animals (Hauptman et al., 1997, Nierhaus et al., 2013, Mare et al., 2015). Recent evidence suggests an association between increased blood concentrations of IGs and a diagnosis of sepsis in human beings (Nierhaus et al., 2013, Mare et al., 2015). In addition, increased concentrations of IGs have been associated with increased disease severity, progression and poor prognosis in different settings of human sepsis (Mare et al., 2015). IGs have altered rheological properties compared to mature cells (Poschl et al., 2005) and, due to poor cell membrane deformability, they may accumulate in specific microvasculature sites, promoting endothelial injury, microcirculatory impairment and local organ dysfunction (van Eden et al., 1997, Poschl et al., 2005). Obstruction of small vessels from activated immature leukocytes has been demonstrated in animal models of low perfusion pressure conditions, such as shock and local ischaemia (Hansell et al., 1993), and in children with Gram negative septicaemia (Poschl et al., 2005).

The potential diagnostic and prognostic value of circulating IGs has been recognised in a number of veterinary studies (Segev et al., 2006, Burton et al., 2013, Burton et al., 2014). The presence of a leukocyte degenerative left shift, defined as the number of IGs exceeding the number of mature neutrophils in circulation, has been associated with an increased risk of death or euthanasia in dogs (Burton et al., 2013). Although the definition of a degenerative left shift is still questionable in respect of the overall number of mature neutrophils to consider (Stockham and Scott, 2008), these data suggest that the presence of IGs may have clinical relevance in diseased animals.

The presence and magnitude of a degenerative left shift may be associated with worse outcomes in cats (Burton et al., 2014). In a prospective study, cats with sepsis were more likely to have severe left shift compared to cats with non-infectious inflammation, although no association between IGs and outcome was identified (DeClue et al., 2011). However, although assessment of circulating IGs by manual evaluation of blood smears currently represents the gold standard for leukocyte classification in veterinary medicine, it can be operator dependent and time consuming (Stockham and Scott, 2008, Park et al., 2011).

The delta neutrophil index (DNI) represents the fraction of circulating IGs in the peripheral blood; it is automatically calculated by the ADVIA-series haematological analysers as the leukocyte difference between the myeloperoxidase channel and the nuclear lobularity channel count (Nahm et al., 2008). The DNI has been associated with positive blood cultures, disseminated intravascular coagulation scores, progression of disease and mortality in septic adult and neonatal human patients (Nahm et al., 2008, Park et al., 2011, Lee et al., 2013, Kim et al., 2014). The DNI also appears to be helpful in identifying human patients with an impending risk of organ dysfunction and septic shock (Park et al., 2011).

No data regarding the diagnostic performance of the DNI have been reported in veterinary medicine. The primary aim of the current study was to evaluate the diagnostic and prognostic significance of the DNI in canine sepsis. A specific preliminary aim was to determine the DNI reference interval (RI) in a population of healthy control dogs.