**7.1. Fluorescent spot test**

Fluorescence is a form of luminescence that uses the physical change of emission of light upon excitation of molecules. There are various different types of luminescence, classified depending on the style of excitation: chemo-luminescence (ending in a chemical reaction) photo-luminescence (fluorescence, phosphorescence and delayed fluorescence), bioluminescence (via a living organism) and others (Bernard, 2002).

Nicotinamide Adenine Dinucleotide Phosphate (NADPH) is the reduced form of NADP, with absorption maximum at 340 nm and a maximum emission at 460 nm. NADPH concentrations have been studied in great detail using optical methods. A parameter for direct measurements of the G6PD activity is the fluorescence of NADPH. When G6PD shows enough functional activity in erythrocytes, two molecules of NADP+ are reduced to NADPH. After the addition of glucose 6-phosphate and NADP+, blood spot fluoresces at *340 nm* if NADPH is produced (Beutler & Baluda, 1966).
