Compare feed TDS and RO TDS under steady-state flow. A rising rejection percentage problem is more informative than the absolute TDS number alone.
Membrane aging or fouling
RO membranes lose performance over time and can foul from hardness, iron or other feed-water conditions.
Seal or bypass leakage
An incorrectly seated membrane, damaged O-ring or internal bypass can mix feed water into the permeate stream.
Pressure and temperature matter
Low pressure and cold water reduce membrane production and can change apparent performance. Compare measurements under similar conditions.
Do not use TDS as a safety verdict
TDS is useful for membrane trending but does not identify lead, PFAS, arsenic or other individual contaminants.
How to verify treatment claims for Why Is My RO Water TDS Suddenly High?
Start with the contaminant or water-quality problem, then verify the exact finished system. Technology labels such as reverse osmosis, activated carbon, ion exchange or ultrafiltration describe a treatment mechanism; they do not prove that every product using that mechanism has the same reduction claims. For health-related contaminants, look for model-specific certification or a performance data sheet tied to the exact unit.
NSF/ANSI 42 commonly covers aesthetic effects such as chlorine taste and odor, NSF/ANSI 53 covers specified health-effect reductions, and NSF/ANSI 58 applies to reverse-osmosis systems. The presence of a standard number does not mean every optional contaminant claim in that standard was certified, so the individual listing still matters.
Flow, installation and three-year ownership cost
A filter that treats the right contaminant can still be a poor fit if the household dislikes its flow rate, reservoir workflow or installation requirements. Compare faucet flow, daily production, drain requirements, available cabinet or counter space and whether a powered pump or dedicated faucet is needed. Renters should give extra weight to reversible installations.
Then annualize the consumables. Add prefilters, membranes, postfilters or proprietary cartridges over at least three years and consider availability where you live. Higher-priced hardware can be the better value when filters last longer and remain easy to source; inexpensive hardware can become costly when several proprietary cartridges need frequent replacement.