GSTM1 and anemia: In addition, numerous non-invasive technologies and tools can indirectly determine the Hb level in blood and the oxygen content in human tissues, such as photoplethysmography (PPG) based on the oral mucosa and conjunctival tissues (15), reflectance spectroscopy of finger and palpebral conjunctiva (16), and fluorescence spectroscopy (17), which are also reported in some studies of anemia prediction.