te Witt R, Hassing R, Petit PP, van Belkum A, van Genderen PJ. Procalcitonin and neopterin levels do not accurately distinguish bacterial from viral infections in ill-returned travellers with fever. Transactions of the Royal Society of Tropical Medicine and Hygiene. 2012 Apr;106(4):264–6. doi: 10.1016/j.trstmh.2012.01.001.