A Sodium Rhodizonate-Based Colorimetric Cassette for Point-of-Care Blood Lead Level Screening: Proof-of-Concept Validation in Synthetic Serum.

Point-of-care testing (POCT) for blood lead level (BLL) remains an unmet diagnostic priority in low- and middle-income countries. A 2022–2024 icddr,b surveillance study reported that 98% of 500 children in Dhaka aged 2–4 years exceeded the CDC reference value of 3.5 µg/dL, with a median BLL of 6.7 µg/dL. Conventional gold-standard methods (ICP-MS, GFAAS) are inaccessible at the community level, leaving millions of children undiagnosed. This study describes the development and validation of the RhodoLead™ cassette: a sodium rhodizonate-based colorimetric POCT device for naked-eye, semi-quantitative blood lead detection without instrumentation. The device exploits the red-violet complex formed between sodium rhodizonate and free ionic lead (Pb²⁺). A mild acid dissociation pre-treatment liberates matrix-bound lead from erythrocytes and plasma proteins, converting it into detectable free Pb²⁺ — ensuring the cassette result reflects total blood lead burden rather than free-ion concentration alone. To suppress false-positive signals from interfering physiological cations (Ca²⁺, Fe³⁺, Cu²⁺), Nitrilotriacetic acid (NTA, 100 µM) was optimised as a selective masking chelant, preserving assay specificity in a complex biological matrix. Validation in synthetic serum across 0–7 µg/dL confirmed strong dose-response relationships by UV-Vis spectrophotometry (480 nm) and a dispersion-length assay. The analytical limit of detection was estimated at 2.5 µg/dL — below both the WHO (5 µg/dL) and CDC (3.5 µg/dL) action thresholds. The cassette requires only 10–30 µL of finger-prick blood, delivers a result within 15 minutes, and costs an estimated $0.30–0.50 per test, supporting a scalable diagnostic pathway for lead poisoning screening in resource-limited settings.