Adult-onset Still's disease (AOSD) is a systemic inflammatory disorder of unknown etiology.1 Several classification criteria for AOSD have so far been suggested, however, there are no objective laboratory tests for its diagnosis.2,3 Thus, diagnosis of AOSD is still based on heterogeneous clinical manifestations and laboratory results with exclusion of other febrile conditions.1 Especially in early phase, it is difficult to differentiate AOSD from infection due to their indistinguishable symptoms and laboratory findings, such as fever and leukocytosis. In order to distinguish the two diseases in early phase, several efforts have so far been made, however, a specific biomarker has not yet been established.4,5,6 Novel biomarkers that could represent the disease activities had been studied in other autoimmune inflammatory diseases.7

Delta neutrophil index (DNI) is an automatically calculated parameter that reflects the ratio of immature granulocytes (IGs) over total neutrophil count in the peripheral circulation 8.8,9 IGs are hematologic cells that do not reach full maturation of complete neutrophil nuclear lobulation,10 and are usually released to peripheral blood by catecholamines and pro-inflammatory cytokines in sepsis.11 DNI has been reported to be more predictive of sepsis severity and prognosis than traditional marker including white blood cell (WBC) counts, absolute neutrophil counts and C-reactive protein (CRP).8,9,12,13 With these reasons, DNI may be considered useful for discriminating AOSD from sepsis, but its clinical application has not been reported to date. In this study, therefore, we investigated clinical implications of DNI for discriminating the two diseases in early phase.

We retrospectively reviewed the medical records of 16 patients with AOSD with 19 episodes of fever at admission. They were admitted for fever at Yonsei University Medical Center from Jan 2010 to Mar 2012. Three patients were excluded because they had no febrile events at admission, and three episodes were excluded because microbial infections were confirmed through culture studies. All patients fulfilled Yamaguchi's criteria for AOSD,3 and had febrile episode at admission. We also reviewed the medical records of 33 gender- and age-matched patients who were admitted to the same institute during the same period, and who were diagnosed with sepsis, excluding hematologic disorder. Sepsis is defined as a systemic inflammatory response syndrome (SIRS) accompanied by proven microbial infection. SIRS was defined as two or more of the following conditions: 1) body temperature >38℃ or <36℃; 2) leukocytosis >12000/µL, leucopenia (<4000/µL); 3) heart rate >90 beats/min; 4) respiratory rate >24/min.8 Bacteremia was defined as the presence of microbial organisms in blood culture. This study was approved by the Institutional Review Board of Severance Hospital.

AOSD is generally a mild and self-limiting disease, and it could progress to a chronic course with relapse and occasional life-threatening complications in one-third of patients. Kim, et al.14 indicated that short diagnostic delay was a good prognostic factor as a result of effective treatment, and response to glucocorticoid therapy was also an important prognostic factor. Moreover, mortality rate of AOSD was 9-10%, and main cause of death was sepsis, pneumonia and disease progression.14,15,16 Especially in sepsis, early recognition and early antibiotics therapy within the first six hours significantly decreased in-hospital mortality.17 Therefore, early differential diagnosis AOSD from sepsis might be a crucial for leading to a good prognosis.14,17,18 Nonetheless, in the clinical settings, and from the clinical point of view, there are several difficulties as follows: 1) it is difficult to distinguish between AOSD and sepsis in the early phase before the report on the results of culture studies; 2) treatment strategies are different between the two diseases; and 3) the long incubation period of culture studies hinders a rapid final diagnosis.

Several biomarkers for the differential diagnosis of the two diseases have so far been suggested, but definite biomarker had not yet been established clinically. Calprotectin produced by monocytes could not distinguish AOSD from microbial infection, despite good correlations with AOSD activity.4,5 And serum levels of cytokines involved in pathophysiology of AOSD did not show significant differences between the two diseases either.4 In addition, hyperferritinemia has been suggested as a most useful marker of AOSD,6 and hyperferritinemia with low glycosylated ferritin had been reported as a more powerful marker. However, they were also insufficient to differentiate AOSD from sepsis because elevated ferritin was often observed in other inflammatory diseases including sepsis and it might not be elevated in the early phase of AOSD.19 Fautrel, et al.20 could not show significant differences of hyperferritinemia with low glycosylated ferritin between AOSD and infection. Previous study suggested that only clinical manifestation could differentiate AOSD from sepsis.4 We compared clinical characteristics between two groups and clinical manifestations, including typical rash and arthralgia, could differentiate between two diseases, but nonspecific symptoms such as lymphadenopathy were often present in patients with sepsis in this study.

Thus, a useful serologic marker is needed to discriminate AOSD from sepsis. We introduced a novel value, DNI for the differential diagnosis of AOSD from sepsis. In the present study, we first verified that DNI in AOSD was significantly lower than in sepsis. One of typical feature of AOSD is leukocytosis with neutrophilia, which was similar to sepsis. The mechanism of leukocytosis remains uncertain although a previous study reported that granulocyte hyperplasia is found in the bone marrow of AOSD patients.21 However, over 80% of neutrophil are mature PMN in AOSD,18 which might be responsible for our results.

Previous studies showed that DNI was early prognostic factor for sepsis when they measured DNI within 24 hours of the onset of SIRS or within the first 24 hours of intensive care unit admission.12,13 With this reason, we analyzed mean DNI during the first 3 days of hospitalization and speculated that DNI could give a supplementary clue for the differential diagnosis between AOSD and sepsis in the early phase before confirming the result of culture studies. We demonstrated in the present study that DNI in patients with AOSD was markedly lower than in patients with sepsis. Moreover, DNIs in three patients who were admitted for AOSD flare were compared to those with confirmed microbial infection at admission, and DNIs of sepsis were found higher than those of AOSD flare in all three patients; 11.7%, 9.2%, and 3.9% vs. 1.2%, 2.8%, and 2.6% (data not shown).

Next, we also divided sepsis patients into two sub-groups depending on the presence of bacteremia, since DNI was reported to be associated with positive blood culture rates in suspected sepsis.8 Contrary to the above results, mean DNI in sepsis patients with bacteremia was slightly higher than in those without, but both mean DNIs of two subgroups were significantly higher than those in AOSD group. These results suggested that DNI could distinguish AOSD from sepsis in the early phase, regardless of the presence of bacteremia.

In addition to DNI, albumin, total bilirubin, and ferritin levels were also significantly different between the two groups in univariate analysis. When we analyzed variables, except ferritin, by using binary logistic analysis, DNI was an independent factor for predicting sepsis. However, in the multivariate analysis including ferritin, the usefulness of DNI for predicting sepsis did not surpass that of ferritin level. We compared AUROC between DNI and ferritin because hyperferritinemia was the most reliable biomarker for diagnosis of AOSD to date. Consequently, we identified that DNI had slightly higher AUC than that of ferritin despite of no statistical significance.

To evaluate the weight of the clinical implication of DNI for differentiating AOSD and sepsis, we set the cut-off value of DNI for differentiating AOSD and sepsis at 2.75%, with a sensitivity of 82.1% and a specificity of 84.6%, and found that its relative risk for sepsis was 176. In previous study that compared patients with sepsis and healthy controls, the cut-off value for sepsis was 2.7%, similar to our result.13 DNI cannot be used for differentiation AOSD from other inflammatory disease, except infection, because elevated DNI is specific for sepsis. However, we have the possibility that DNI can be a reliable marker for distinguishing between AOSD and sepsis that have relatively high sensitivity and specificity. Also, DNI is included in the complete blood count test, which is routinely performed without additional costs or tests when patients with fever are admitted to a hospital. Considering that there are only a few objective laboratory results to help physicians to discriminate AOSD and sepsis in their early phase, DNI can be a raid and efficious marker for it. Consequently, we suggest that DNI might be useful to differentiate AOSD from sepsis as a supplementary variable to ferritin, although DNI could not be an independent predictive value for differential diagnosis between the two diseases.

This study has two limitations: first, small number of AOSD patients was enrolled in the study due to its rare incidence, and second, other biomarkers such as procalcitonin or calprotectin were not analyzed. If future study includes larger number of AOSD patients and measure these biomarkers together, it might reveal a dynamic role for differential diagnosis of AOSD from sepsis in early phase.

In conclusion, we suggest that DNI might be a useful marker for differential diagnosis between AOSD from sepsis at an early phase of the two diseases when their disease patterns are indistinguishable.