Trying to understand tds's

[Silverni2]

My ro/di filters are 2 years old need to be changed. I test my tds with a new meter I just purchased, they test at 15. My nitrate and phosphate from the ro/di r undetectable with salifert test kit. Do the tds breakdown into po4 and no3? If not why worry about tds?
Thx

 

[mbg75]

TDS meter measures conductivity. Like salts, metals etc.
Organic compounds aren't always detected by TDS.

My TDS read 0 recently. But my phosphates was .1