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Early detection and diagnosis of diseases is a critical issue. A colorimetric “point-of-care” device – based on the principle of electrochromism – is proposed here for the precocious diagnosis of cystic fibrosis, especially in resource-limited environments, developing countries and ambulatory contests. We designed a complete device architecture to detect sodium chloride in a small amount of human sweat (3 μL), conceived in order to complete the steps of sample preparation and disease diagnosis. The device, as an alternative route for conductivimetric analysis, measures the amount of sodium chloride in a sample of sweat exploiting the electrochromic properties of tungsten oxide. In our case, sodium sweat promotes an oxide blue coloration depending on the cation concentration. Indeed the device shows an effective transmittance modulation when sodium cations exceed the physiological threshold. optical modulations of 48%, 35%, 15%, 7% were observed, at 555 nm, for sodium concentrations of 120 mmol/L, 90 mmol/L, 60 mmol/L, and 30 mmol/L, respectively. In the reported experimental investigations, both human and artificial sweat were employed, with strictly comparable results. [ABSTRACT FROM AUTHOR]