From our OMA:"Contaminated runway" means a runway of which more than 25% of the runway surface area within the required length and width being used is covered by the following:
- surface water more than 3 mm (0.125 in) deep, or by slush, or loose snow, equivalent to more than 3 mm (0.125 in) of water;
- snow which has been compressed into a solid mass which resists further compression and will hold together or break into lumps if picked up (compacted snow); or
- ice, including wet ice.
As a general rule it's safer to use the most limiting conditions as ATIS reports are not always accurate in terms of time, consider the braking actions as a barometer and pilot reports from similar aircraft types.
Worth mentioning that landing performance and take off performance may be different for the same runway and our old ops manual stated with braking action less than "medium" it was a divert/hold etc.