Interpreting Lab Results
Lab reports are designed for scientists. But the core concepts are straightforward once you know five abbreviations.
ND (Not Detected)
ND means the analyte was below the method detection limit (MDL). Not zero -- the instrument could not measure it at the sensitivity used. For contaminants, ND is the best result.
Check the MDL value. ND with MDL of 1 ppb is very different from ND with MDL of 50 ppb. Lower MDL means more certainty the substance is truly absent.
MCL (Maximum Contaminant Level)
EPA-set legal limits for public water. Represents maximum allowed, not safe concentration. Many MCLs were set decades ago based on treatment feasibility, not health science.
MCLG is the health-based target, often lower. Lead MCLG: 0. Arsenic MCLG: 0. Enforceable arsenic MCL: 10 ppb -- a compromise between health and cost.
Units
ppm = mg/L. 1 part per million.
ppb = ug/L. 1 ppb = 0.001 ppm. Used for trace contaminants (lead, arsenic, PFAS).
ppt = parts per trillion. 1 ppt = 0.001 ppb. Used for PFAS. EPA PFOA advisory: 0.004 ppt.
Method detection limits
MDL is the lowest concentration a method reliably distinguishes from zero. Reporting limit (RL) is typically 3-10x the MDL. Below RL, results are reported as ND.
A lab using EPA 200.8 for lead might have MDL of 0.5 ppb. Another lab: MDL of 5 ppb. Both report ND for the same sample, but the first gives 10x more certainty.
Quick reference
ND: not detected -- best result for contaminants
MCL: legal limit, not safety limit
MCLG: health-based goal, often stricter
1 ppm = 1,000 ppb = 1,000,000 ppt
Lower MDL = higher confidence in ND results