What is NEUT (neutrophil-to-lymphocyte ratio) in a patient with Systemic Lupus Erythematosus (SLE) presenting with fever?

What is NEUT in SLE with Fever?

NEUT refers to neutrophil-related parameters on automated hematology analyzers, including absolute neutrophil count and derived indices like the neutrophil-to-lymphocyte ratio (NLR), which serve as critical biomarkers for distinguishing infection from disease flare in febrile SLE patients.

Understanding NEUT Parameters

NEUT encompasses several measurements from automated cell counters:

• Absolute neutrophil count: The total number of neutrophils in circulation 1
• NLR (neutrophil-to-lymphocyte ratio): Calculated by dividing absolute neutrophil count by absolute lymphocyte count 1, 2, 3
• NEUT-x, NEUT-y, NEUT-z indices: Automated analyzer parameters that reflect neutrophil characteristics and maturity 1

Clinical Significance in Febrile SLE

Distinguishing Infection from Flare

When an SLE patient presents with fever, elevated NEUT parameters (particularly NLR ≥5.70-6.3) strongly suggest bacterial infection rather than lupus flare alone 1, 2, 4:

• Infection pattern: High total leukocyte count, high NLR (≥6.3), elevated NEUT-x, NEUT-y, and NEUT-z indices 1
• Flare pattern: High SLEDAI-2K score, elevated anti-dsDNA, low complement (C3, C4), but lower NLR 1

Specific Diagnostic Thresholds

The most robust evidence supports these cut-off values:

• NLR ≥6.3: Sensitivity 70%, specificity 85%, PPV 83%, NPV 74% for non-viral infections 2
• NLR ≥5.70: Sensitivity 75%, specificity 90% for any infection 4
• Combined NLR + CRP (≥7.5 mg/L): Specificity increases to 90-98%, PPV 88% 2

Practical Clinical Algorithm

When evaluating fever in SLE:

1. Obtain complete blood count with differential to calculate NLR immediately 1, 2
2. If NLR <2.8-3.0: Strongly favors lupus flare over infection 1, 3
3. If NLR ≥6.0: Pursue infectious workup aggressively (cultures, imaging, antimicrobials) 2, 4
4. If NLR 3.0-6.0: Add CRP measurement—if CRP ≥7.5 mg/L, infection is likely 2
5. Check complement levels and anti-dsDNA: Low C3/C4 with high anti-dsDNA suggests concurrent flare 1

Important Caveats

Baseline Hematologic Abnormalities in SLE

Be aware that SLE patients often have baseline cytopenias 5, 6:

• Severe lymphopenia (≤1×10⁹/L or <500 cells/mm³) increases infection risk independent of NLR 5, 6
• Autoimmune leukopenia is common in SLE but rarely requires treatment 6
• Drug-induced leukopenia from immunosuppressives must be excluded when interpreting low counts 6

CRP Interpretation

CRP >50 mg/L in SLE strongly suggests superimposed infection rather than disease activity alone, as SLE flares typically do not elevate CRP significantly 5, 6:

• Many SLE patients have blunted CRP responses during pure disease flares 5, 6
• Marked CRP elevation is a red flag for bacterial infection 5, 6

Disease Activity Monitoring

Beyond infection diagnosis, NLR correlates with SLE disease activity:

• Higher NLR associates with higher SLEDAI scores (r = 0.237, p = 0.01) 3
• NLR ≥2.293 identifies high disease activity with 68.9% sensitivity and 82.8% specificity 3
• However, this relationship is weaker than NLR's utility for infection diagnosis 3

Cost-Effective Approach

A validated model using age, total leukocyte count, and CRP achieves AUC 0.88 for discriminating infection from flare, making it practical for resource-constrained settings 1:

• This combination uses routinely available, low-cost parameters 1
• Procalcitonin adds minimal discriminatory value beyond NLR and CRP 1
• NEUT-z combined with C4 shows promise but requires further validation 1