Study Population and Sample Collection

Ten schools were randomly selected from a list of all high schools in Puerto Princesa. All students attending one of the selected schools were screened for eligibility. A randomly selected subset of all students aged 12 years and above who did not suffer from acute or chronic illnesses were invited to participate. After the participants gave written assent and their parents/legal guardians provided written informed consent, information on sex and age was collected. Two hundred fifty microliters of capillary blood and 3 mL of venous blood were collected in an EDTA microtainer and EDTA vacutainer, respectively. All samples were stored at 4–8°C. Capillary blood was processed within 48 hours of collection while 73% of the venous blood samples were analyzed within two days after collection and the rest in 3 to 6 days. The stability of the samples was not tested over six days of storage.