Fibrinogen is a blood protein that is essential for clotting. It is converted into the polymer fibrin by the blood enzymes thrombin and factor XIIIa. Fibrinogen is one of the first proteins to be depleted in heavily bleeding patients. Patients with early hypofibrinogenemia need urgent fibrinogen replenishment to prevent the onset of haemorrhage and death. However, currently there is no rapid, sensitive, cheap and easy-to-use fibrinogen assay that can detect fibrinogen concentrations. In this study, we have developed a new paper-based diagnostic to quantify the fibrinogen concentration in blood at room temperature. This diagnostic is a 2-step process: first, plasma is added onto thrombin-treated paper strips where fibrinogen is converted to fibrin; then the strips are placed into an aqueous dye bath where elution occurs. The test operates by measuring the change in hydrophobicity, which increases with fibrinogen concentration under otherwise constant conditions. The diagnostic can precisely measure fibrinogen concentration within the range of 0-2 g L-1, which is ideal for the clinical diagnosis of hypofibrinogenemia. Furthermore, testing needs only 12 μL of plasma, 60 mU of thrombin and 7.5 minutes of testing. This diagnostic has the potential to revolutionise point of care testing and save many lives.