This is a prospective observational study of patients presenting to ED of Prince of Wales Hospital (PWH). Patients aged 18 or above who are triage category 3 presenting to ED will be recruited. Capillary and venous blood samples will be collected. Venous lactate levels will be measured by blood gas analyzer (Siemens Automatic QC RAPID Systems RAPID Point@500) and two hand-held lactate analyzers (StatStrip Xpress Lactate Meter and Lactate Scout + ). Venous lactate levels measured by blood gas analyzer will be served as "reference method". Capillary lactate levels will be analyzed by handheld lactate analyzer. We are aimed to recruited 240 patients.