TDS is useful for tracking RO membrane performance and broad mineral load. It is not a contaminant safety test.
What TDS includes
Total dissolved solids can include calcium, magnesium, sodium, bicarbonate and many other dissolved ions. Some are harmless or desirable at normal levels; others can be contaminants.
Two samples can have the same TDS and completely different chemistry.
Why a cheap TDS meter is limited
Most handheld TDS meters estimate dissolved solids from electrical conductivity. They do not directly weigh dissolved material or identify individual contaminants.
A low reading cannot tell you whether lead, PFAS, arsenic or another specific contaminant is absent.
Where TDS is genuinely useful
TDS is useful for comparing feed water with RO permeate and spotting a major change in membrane rejection. If an RO system that normally produces very low-TDS water suddenly rises substantially, the membrane or system may need attention.
Use percentage rejection as a trend, not as proof of removal for every contaminant.
TDS and taste
Mineral content affects taste. Very low-TDS RO water can taste flat to some people, which is why some systems add remineralization after the membrane.
Taste preference is separate from contaminant performance.