As it's easy and cheap, you have no reason to use a measurement in isolation. For instance, you can track it every day to see if the infection is growing or shrinking. Here's a recent example regarding COVID-19: "We realized that whereas a single CRP lab value from hospital admission wasn't very practical as a predictor of who might get sicker, tracking the rate of change from Day 1 to Day 2 or 3 was a very powerful and very clinically predictive test" https://www.sciencedaily.com/releases/2020/11/201105112947.h...