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Background: This case conference reviews laboratory methods for assessing oxygenation status: arterial blood gases, pulse oximetry, and CO-oximetry. Caveats of these measurements are discussed in the context of two methemoglobinemia cases. Cases: Case 1 is a woman who presented with increased shortness of breath, productive cough, chest pain, nausea, fever, and cyanosis. CO-oximetry indicated a carboxyhemoglobin (COHb) fraction of 24.9%. She was unresponsive to O 2 therapy, and no source of carbon monoxide could be noted. Case 2 is a man who presented with syncope, chest tightness, and signs of cyanosis. His arterial blood was dark brown, and COoximetry showed a methemoglobin (MetHb) fraction of 23%. Issues: Oxygen saturation (SO 2 ) can be measured by three approaches that are often used interchangeably, although the measured systems are quite different. Pulse oximetry is a noninvasive, spectrophotometric method to determine arterial oxygen saturation (S a O 2 ). CO-oximetry is a more complex and reliable method that measures the concentration of hemoglobin derivatives in the blood from which various quantities such as hemoglobin derivative fractions, total hemoglobin, and saturation are calculated. Blood gas instruments calculate the estimated O 2 saturation from empirical equations using pH and PO 2 values. In most patients, the results from these methods will be virtually identical, but in cases of increased dyshemoglobin fractions, including methemoglobinemia, it is crucial that the distinctions and limitations of these methods be understood. Conclusions: SO 2 calculated from pH and PO 2 should be interpreted with caution as the algorithms used assume normal O 2 affinity, normal 2,3-diphosphoglycerate con-

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Direct electron transfer of horse heart cytochrome c is measured at a nanocrystalline boron-doped diamond thin-film electrode. A quasi-reversible, diffusion-controlled cyclic voltammetric response is observed for untreated diamond. The peak currents change linearly with the concentration, and importantly, there is no electrode fouling. The results, observed for a hydrogen-terminated and uncharged surface, (i.e., no ionizable carbon-oxygen functional groups), raise interesting questions about the necessary surface interactions of cytochrome c for relatively rapid electrode kinetics.

The concentration of chloride in sweat remains the most robust biomarker for confirmatory diagnosis of cystic fibrosis (CF), a common life-shortening genetic disorder. Early diagnosis via quantitative assessment of sweat chloride allows prompt initiation of care and is critically important to extend life expectancy and improve quality of life. The collection and analysis of sweat using conventional wrist-strapped devices and iontophoresis can be cumbersome, particularly for infants with fragile skin, who often have insufficient sweat production. Here, we introduce a soft, epidermal microfluidic device (“sweat sticker”) designed for the simple and rapid collection and analysis of sweat. Intimate, conformal coupling with the skin supports nearly perfect efficiency in sweat collection without leakage. Real-time image analysis of chloride reagents allows for quantitative assessment of chloride concentrations using a smartphone camera, without requiring extraction of sweat or external analysis. Clinical validation studies involving patients with CF and healthy subjects, across a spectrum of age groups, support clinical equivalence compared to existing device platforms in terms of accuracy and demonstrate meaningful reductions in rates of leakage. The wearable microfluidic technologies and smartphone-based analytics reported here establish the foundation for diagnosis of CF outside of clinical settings.