Summing nondetects: incorporating low-level contaminants in risk assessment

Low-level contaminants often are present below the detection or reporting limits of a laboratory, resulting in values reported as a nondetect or less-than. How can these values be summed along with detected concentrations to obtain a total, particularly when weighting factors such as toxic equivalence factors (TEFs) are used? The most common method employed by environmental scientists for summing nondetects along with detected values is to substitute one-half the detection limit for each nondetect. This substitution allows the least precise measurements, data with high detection limits, to have a strong influence on the resulting total amount. Substitution methods have repeatedly been shown to provide substandard results in studies over the last 2 decades. Here an alternative, the Kaplan-Meier (KM) method used throughout the fields of medical and industrial statistics, is used to obtain the total. KM estimates are far less affected by the least precise data than are estimates computed using substitution. No assumptions about the distribution of data (whether they follow a normal or other distribution) need be made. Direct application of KM to computation of toxicity equivalence concentrations (TECs) is shown.