Dengue fever is the most rapidly spreading arthropod-borne viral infection in the world, transmitted to humans principally by the bite of infected female Aedes aegypti and Aedes albopictus mosquitoes.1 The global incidence of dengue has risen dramatically in recent decades: cases reported to the World Health Organization increased from just over half a million in the year 2000 to 14.6 million in 2016, the highest annual total ever recorded, with transmission now documented in more than 100 countries across all six WHO regions.2 A global burden-of-disease analysis similarly estimated that dengue incidence more than doubled between 1990 and 2016, rising from 26.45 million to 58.96 million cases annually, accompanied by a parallel rise in dengue-attributable deaths and disability-adjusted life years.3 Clinically, dengue infection ranges from an asymptomatic or mild self-limiting febrile illness to a life-threatening syndrome characterised by plasma leakage, severe bleeding, and organ impairment. The WHO 2009 classification stratifies patients into dengue without warning signs, dengue with warning signs, and severe dengue, the last requiring evidence of severe plasma leakage, severe haemorrhage, or significant organ involvement.4 Because the early clinical features of dengue, fever, headache, myalgia, and retro-orbital pain, closely mimic those of other acute febrile illnesses prevalent in endemic regions, and because confirmatory serological or molecular testing is often costly, time-consuming, or unavailable at the point of care, clinicians have long sought simple, rapid, and widely accessible laboratory tools to support early diagnosis and risk stratification, including composite severity scores that incorporate routine clinical and haematological profiles.5 The complete blood count has emerged as one of the most useful such tools. Dengue virus infection produces a well-described and temporally evolving pattern of haematological change: monocytosis appears earliest in the course of fever, followed sequentially by leukopenia, thrombocytopenia, haemoconcentration due to capillary plasma leakage, the appearance of atypical reactive lymphocytes, and a reversal of the neutrophil-to-lymphocyte ratio, with most abnormalities becoming evident around the third to fifth day of illness and resolving by the tenth or eleventh day.6,9 These changes are thought to reflect both direct viral suppression of bone marrow megakaryocyte and myeloid precursors and peripheral immune-mediated destruction, including increased binding of platelets and neutrophils to dengue-infected vascular endothelial cells.6 Thrombocytopenia is among the most consistently reported and clinically important haematological abnormalities in dengue, with platelet counts falling progressively as disease severity increases and reaching their nadir in patients who develop severe dengue or dengue with warning signs.7 Leukopenia, too, has repeatedly been shown to track with disease severity and has been proposed as an early, low-cost marker that may even outperform thrombocytopenia in some populations.8 A rising haematocrit, reflecting plasma leakage from increased vascular permeability, is the haematological hallmark of evolving severe dengue, and the WHO explicitly recommends serial haematocrit monitoring during the critical phase of illness, with a rise of 20% or more above the patient's baseline considered a warning sign for impending shock.4,9 When thrombocytopenia, leukopenia, and a rising haematocrit occur together, their combined presence has been reported to carry particularly strong diagnostic and prognostic significance.9 Other parameters, including reduced haemoglobin, altered mean platelet volume, raised red cell distribution width, and markedly elevated total leukocyte counts in the setting of secondary bacterial infection or severe systemic inflammation, have also been linked to adverse outcomes including prolonged hospitalisation, need for transfusion, and mortality.10 Despite the wealth of literature on individual haematological parameters in dengue, their relative diagnostic and prognostic value, and the precise pattern and magnitude of change across WHO severity categories, continue to vary across populations, study settings, and the prevailing circulating dengue serotype. Given the renewed global surge in dengue transmission and the practical need for accessible, low-cost tools to guide triage in resource-constrained settings, continued evaluation of haematological parameters across diverse patient populations remains clinically relevant.2,3 This study was therefore undertaken to systematically evaluate haemoglobin, leukocyte indices, platelet count, and haematocrit in patients with serologically confirmed dengue fever, and to examine how these parameters vary across WHO-defined categories of disease severity.

Study Design and Setting This was a hospital-based, cross-sectional observational study conducted jointly in the Department of Pathology and the Department of General Medicine of a tertiary care teaching hospital. Patients were enrolled consecutively over a twelve-month period coinciding with the local dengue transmission season. Study Population All patients of either sex presenting with acute febrile illness and a positive dengue NS1 antigen test and/or dengue-specific IgM/IgG enzyme-linked immunosorbent assay (ELISA) were screened for eligibility. Inclusion criteria were: age ≥12 years; acute fever of ≤7 days' duration at presentation; and laboratory confirmation of dengue infection by NS1 antigen positivity, dengue IgM positivity, or both. Exclusion criteria were: a known pre-existing haematological disorder (e.g., haemoglobinopathy, chronic leukaemia, myelodysplastic syndrome); chronic liver or renal disease; pregnancy; recent blood transfusion within the preceding four weeks; concurrent malaria, typhoid, leptospirosis, or other confirmed co-infection; and incomplete clinical or laboratory records. Data Collection At admission, demographic data (age, sex), day of illness, and clinical features (fever, myalgia, headache, rash, bleeding manifestations, abdominal pain, vomiting) were recorded on a structured proforma. Venous blood samples were collected into ethylenediaminetetraacetic acid (EDTA) vacutainers at presentation and processed within one hour on a five-part automated haematology analyser, calibrated and quality-controlled according to standard laboratory protocols. Parameters recorded included haemoglobin (Hb), total leukocyte count (TLC), differential leukocyte count (neutrophils, lymphocytes, monocytes, eosinophils), absolute platelet count, haematocrit (HCT), mean platelet volume (MPV), and red cell distribution width (RDW). Peripheral blood smears were examined by a haematopathologist to confirm platelet counts in cases of marked thrombocytopenia and to identify atypical reactive lymphocytes. Case Definitions and Severity Classification Disease severity was classified according to the WHO 2009 dengue case classification into three categories: dengue without warning signs (DWoWS), dengue with warning signs (DWWS, e.g., abdominal pain, persistent vomiting, clinical fluid accumulation, mucosal bleeding, lethargy, liver enlargement, or rising haematocrit with concurrent rapid fall in platelet count), and severe dengue (SD, defined by severe plasma leakage leading to shock or fluid accumulation with respiratory distress, severe bleeding, or severe organ impairment). Thrombocytopenia was defined as a platelet count <150 × 10⁹/L; leukopenia was defined as a total leukocyte count <4.0 × 10⁹/L; and a significant rise in haematocrit was defined as an increase of ≥20% above the patient's known or estimated baseline. Statistical Analysis Data were entered into a statistical software spreadsheet and analysed using standard statistical software. Continuous variables were expressed as mean ± standard deviation and compared across the three severity groups using one-way analysis of variance (ANOVA) with post-hoc testing where appropriate. Categorical variables were expressed as frequencies and percentages and compared using the chi-square test or Fisher's exact test as appropriate. A two-tailed p-value <0.05 was considered statistically significant. The study protocol was approved by the Institutional Ethics Committee, and written informed consent was obtained from all participants (or parental/guardian consent with assent for minors) prior to enrolment.

A total of 246 patients with suspected dengue were screened, of whom 220 met the inclusion criteria and were enrolled in the study. The mean age of the cohort was 32.6 ± 13.8 years (range 12–76 years), and 129 (58.6%) were male. Based on the WHO 2009 classification, 128 patients (58.2%) had dengue without warning signs, 71 patients (32.3%) had dengue with warning signs, and 21 patients (9.5%) had severe dengue. DWoWS = dengue without warning signs; DWWS = dengue with warning signs; SD = severe dengue. Day of illness, mucosal bleeding, persistent vomiting/abdominal pain, and shock/fluid accumulation differed significantly across groups (p<0.001). Table 1. Demographic and clinical profile of the study population stratified by disease severity (N = 220) Characteristic DWoWS (n=128) DWWS (n=71) SD (n=21) Age, years (mean ± SD) 31.2 ± 12.9 33.8 ± 14.6 37.4 ± 15.1 Male sex, n (%) 73 (57.0) 43 (60.6) 13 (61.9) Day of illness at admission (mean ± SD) 3.1 ± 1.2 4.4 ± 1.4 5.2 ± 1.6 Fever, n (%) 128 (100.0) 71 (100.0) 21 (100.0) Myalgia/arthralgia, n (%) 96 (75.0) 54 (76.1) 15 (71.4) Rash, n (%) 31 (24.2) 22 (31.0) 6 (28.6) Mucosal/gum bleeding, n (%) 4 (3.1) 18 (25.4) 11 (52.4) Persistent vomiting/abdominal pain, n (%) 9 (7.0) 39 (54.9) 17 (81.0) Shock/fluid accumulation with distress, n (%) 0 (0.0) 0 (0.0) 14 (66.7) Table 2. Haematological parameters by dengue severity category (mean ± SD) Parameter DWoWS (n=128) DWWS (n=71) SD (n=21) Haemoglobin, g/dL 13.4 ± 1.6 13.1 ± 1.8 12.6 ± 2.1 Haematocrit, % 38.4 ± 4.1 42.7 ± 5.3 47.9 ± 6.0 Total leukocyte count, ×10⁹/L 4.8 ± 1.6 3.6 ± 1.4 3.1 ± 2.5 Neutrophils, % 58.2 ± 9.6 52.4 ± 10.8 60.1 ± 12.3 Lymphocytes, % 33.1 ± 8.4 38.9 ± 9.7 30.5 ± 11.2 Atypical lymphocytes present, n (%) 22 (17.2) 31 (43.7) 13 (61.9) Platelet count, ×10⁹/L 167.4 ± 42.6 88.3 ± 31.5 38.6 ± 18.2 Mean platelet volume, fL 9.8 ± 1.1 10.9 ± 1.3 11.8 ± 1.5 Red cell distribution width, % 13.2 ± 1.1 13.9 ± 1.4 14.8 ± 1.7 DWoWS = dengue without warning signs; DWWS = dengue with warning signs; SD = severe dengue. All parameters differed significantly across the three groups on one-way ANOVA (p<0.001), except neutrophil and lymphocyte percentages, where the difference reflects a U-shaped trend rather than a strict linear gradient. Explanation of Findings Table 1 shows that the three severity groups were broadly comparable in age and sex distribution, but differed markedly in day of illness at presentation and in warning-sign features; patients with severe dengue presented later in the course of fever and had a substantially higher prevalence of mucosal bleeding, persistent vomiting or abdominal pain, and frank shock or fluid accumulation with respiratory distress, consistent with the WHO severity definitions used for classification. Table 3. Frequency of key haematological abnormalities across dengue severity categories Abnormality DWoWS (n=128) DWWS (n=71) SD (n=21) Thrombocytopenia (<150 ×10⁹/L), n (%) 91 (71.1) 71 (100.0) 21 (100.0) Severe thrombocytopenia (<50 ×10⁹/L), n (%) 3 (2.3) 19 (26.8) 17 (81.0) Leukopenia (<4.0 ×10⁹/L), n (%) 57 (44.5) 47 (66.2) 17 (81.0) Haematocrit rise ≥20% from baseline, n (%) 6 (4.7) 29 (40.8) 16 (76.2) Combined triad (thrombocytopenia + leukopenia + rising HCT), n (%) 4 (3.1) 24 (33.8) 15 (71.4) TLC ≥10 ×10⁹/L (marked leukocytosis), n (%) 2 (1.6) 3 (4.2) 4 (19.0) HCT = haematocrit; TLC = total leukocyte count. All comparisons across the three severity groups were statistically significant on chi-square testing (p<0.001). Marked leukocytosis in severe dengue is presumed to reflect a secondary inflammatory or bacterial process rather than dengue infection alone. Table 2 demonstrates a clear, statistically significant gradient in core haematological indices across severity categories. Platelet count fell progressively from a mean of 167.4 × 10⁹/L in dengue without warning signs to 38.6 × 10⁹/L in severe dengue, while haematocrit rose correspondingly, reflecting increasing plasma leakage. Mean platelet volume and red cell distribution width also rose with severity, suggesting increased platelet turnover and a degree of marrow stress response accompanying more severe disease. Atypical lymphocytes, classically associated with the later febrile and defervescence phase of dengue, were markedly more common in dengue with warning signs and severe dengue, in keeping with these patients presenting later in their illness course. Table 3 highlights that thrombocytopenia, although already present in nearly three-quarters of patients with uncomplicated dengue, became universal in dengue with warning signs and severe dengue, while severe thrombocytopenia (<50 × 10⁹/L) was essentially confined to the more severe categories. Leukopenia followed a similar but less steep gradient. Most notably, the combined presence of thrombocytopenia, leukopenia, and a haematocrit rise of 20% or more, the so-called diagnostic triad, was rare in uncomplicated dengue but present in nearly three-quarters of severe dengue cases, underscoring the value of evaluating these parameters together rather than in isolation. A small subset of patients with severe dengue showed marked leukocytosis (TLC ≥10 × 10⁹/L), a pattern that runs counter to the typical leukopenic trend of dengue and may signal secondary bacterial infection or an exaggerated systemic inflammatory response.

This study evaluated a panel of haematological parameters in 220 patients with serologically confirmed dengue fever and demonstrated a clear, statistically significant association between the severity of haematological derangement and clinical disease severity as defined by the WHO 2009 classification. These findings are consistent with, and extend, a substantial body of prior literature on this topic.6,9 Thrombocytopenia was the most prevalent and discriminating abnormality in our cohort, present in over 80% of patients overall and in all patients with severe dengue, with mean platelet counts falling sharply across severity categories. This mirrors findings from a Pakistani cohort in which patients with severe dengue infection had a mean platelet count of approximately 50 × 10⁹/L, significantly lower than in patients with non-severe disease.7 The proposed mechanisms for dengue-associated thrombocytopenia include direct viral suppression of megakaryopoiesis in the bone marrow, increased peripheral destruction of platelets through immune-mediated mechanisms, and enhanced binding of platelets to dengue-infected vascular endothelium.6 Leukopenia was likewise common and increased in frequency with disease severity in our cohort, in agreement with data from Thailand showing that leukopenia, alongside monocytosis, thrombocytopenia, and a rising haematocrit, follows a predictable temporal sequence over the course of dengue illness, generally appearing within the first week of fever and resolving by the tenth or eleventh day.9 A recent cross-sectional study from Nepal similarly identified leukopenia as an early and clinically useful marker of dengue severity, even while raising questions about the independent prognostic value of thrombocytopenia in isolation, a nuance that supports our finding that the combined haematological triad carries stronger discriminative value than any single parameter alone.8 The progressive rise in haematocrit observed across our severity groups, and its strong association with severe dengue, is consistent with the WHO's emphasis on haematocrit as a marker of plasma leakage and impending shock, with a rise of 20% or more above baseline considered a key warning sign warranting closer monitoring and fluid management.4,9 Notably, in our cohort, the combined presence of thrombocytopenia, leukopenia, and a significant haematocrit rise was substantially more discriminating for severe disease than any single parameter, a pattern that has also been highlighted in prior work describing this triad as the most clinically significant haematological finding in dengue.9 Other haematological findings in our study, including a rise in mean platelet volume and red cell distribution width and the emergence of atypical lymphocytes with increasing severity, align with the broader literature describing reactive marrow and lymphocyte responses during the critical and defervescence phases of dengue infection.6,9 The small subset of patients with severe dengue who exhibited marked leukocytosis rather than the expected leukopenia is consistent with reports from paediatric cohorts in which a total leukocyte count of 20 × 10⁹/L or more was strongly and independently associated with mortality, with an odds ratio exceeding eleven, likely reflecting secondary bacterial infection or an overwhelming systemic inflammatory response superimposed on the underlying viral illness.10 This finding carries an important clinical message: a high leukocyte count in a patient with suspected or confirmed dengue should not be reassuring, and instead may signal a complicated or high-risk clinical course requiring closer surveillance. Several limitations of this study should be acknowledged. First, this was a single-centre, cross-sectional study, which captured haematological parameters largely at a single time point at admission rather than longitudinally across the full course of illness; given that dengue-associated haematological changes evolve dynamically over days, serial sampling would have provided a more complete picture of each parameter's trajectory. Second, dengue serotype and viraemia level, both of which may influence the severity and pattern of haematological change, were not determined in this study. Third, as an observational, single-centre study, the generalisability of absolute cut-off values to other populations and circulating serotypes may be limited. Finally, potential confounders such as nutritional status, comorbidities, and concurrent medication use were not systematically captured. Despite these limitations, the consistency of our findings with previously published literature across diverse geographic settings strengthens confidence in the clinical utility of routine haematological monitoring in dengue and supports its continued use as an accessible, low-cost adjunct to clinical assessment, particularly in resource-limited settings where serological or molecular confirmation may be delayed or unavailable.7,8

In this cross-sectional study of patients with confirmed dengue fever, thrombocytopenia, leukopenia, and a rising haematocrit each correlated significantly with increasing clinical severity, and the combined presence of all three abnormalities was strongly associated with severe dengue. Additional indices, including mean platelet volume, red cell distribution width, and the emergence of atypical lymphocytes, provided further supportive evidence of evolving disease severity, while paradoxical marked leukocytosis identified a small but clinically important subgroup at risk of a complicated course. These findings reinforce the value of the complete blood count as a simple, inexpensive, and widely available tool for the early diagnosis, severity stratification, and ongoing monitoring of patients with dengue fever, and support its routine, serial use, particularly in endemic and resource-constrained settings where the global burden of dengue continues to rise. Larger, multicentre, and longitudinal studies incorporating serotype data are warranted to further refine severity-specific haematological thresholds and to validate their predictive performance across diverse populations.

In this cross-sectional study of patients with confirmed dengue fever, thrombocytopenia, leukopenia, and a rising haematocrit each correlated significantly with increasing clinical severity, and the combined presence of all three abnormalities was strongly associated with severe dengue. Additional indices, including mean platelet volume, red cell distribution width, and the emergence of atypical lymphocytes, provided further supportive evidence of evolving disease severity, while paradoxical marked leukocytosis identified a small but clinically important subgroup at risk of a complicated course. These findings reinforce the value of the complete blood count as a simple, inexpensive, and widely available tool for the early diagnosis, severity stratification, and ongoing monitoring of patients with dengue fever, and support its routine, serial use, particularly in endemic and resource-constrained settings where the global burden of dengue continues to rise. Larger, multicentre, and longitudinal studies incorporating serotype data are warranted to further refine severity-specific haematological thresholds and to validate their predictive performance across diverse populations.