Few things that come to mind ...

Few things that come to mind that you need to confirm:

1- Ensure that your sample point is sanitary. Typically stainless steel and it will need to be torched prior to taking the sample. It is very easy to contaminate a sample, so follow sampling guidelines strictly. 

2- Having a residual is not enough information. CT is an important consideration ie. free residual x effective contact time. Make sure you are sampling after a reasonable effective contact time. The germicidal efficacy of chlorine is correlated with the product of the chlorine dose applied or residual (mg/L) and the contact time (min), also referred to as CT. Increasing either the
chlorine residual or the contact time will increase the CT value and the log inactivation of target pathogens. Refer to your applicable guidelines for CT vs inactivation.