Drug testing detects illicit or controlled substances and their breakdown products within a biological sample. Laboratories quantify the concentration of the drug or its metabolites precisely, rather than providing a simple yes or no answer. This measurement uses the standardized unit nanograms per milliliter, abbreviated as ng/ml, which provides the scientific basis for determining a final, legally sound test result.
What Nanograms Per Milliliter Represents
The unit ng/ml is a measurement of concentration, which describes the amount of a specific substance dissolved within a volume of liquid. The “ml” stands for milliliter, a common unit of volume often used for liquids like urine or blood. The “ng” component, or nanogram, represents the mass of the substance being measured.
A nanogram is an incredibly small unit of mass, equal to one billionth of a single gram. The concentration of drug metabolites found in a sample is often so minute that this highly sensitive measurement is required for accurate quantification. Laboratories must measure this concentration precisely because the mere presence of a substance is not enough to declare a positive result; the concentration must exceed a predetermined threshold.
The Critical Role of Cutoff Levels
Detecting trace amounts of a drug metabolite is not sufficient to report a positive test. Drug testing relies on established “cutoff levels” to prevent false results and account for possible environmental or passive exposure. A cutoff level is the minimum concentration, measured in ng/ml, that a drug or its metabolite must meet or exceed for the test to be flagged as non-negative.
These concentration thresholds are standardized based on scientific consensus and often guided by federal regulations. For example, federal guidelines establish the initial screening cutoff for the main marijuana metabolite, carboxy-THC, at 50 ng/ml in a urine sample. Cocaine metabolites often have a higher initial cutoff level, typically set at 300 ng/ml. Setting these limits ensures the testing process is fair and legally defensible by filtering out insignificant trace amounts.
How ng/ml Determines the Final Result
The final determination of a positive or negative result is a two-step process that utilizes different ng/ml cutoffs. The first step involves a quick, cost-effective initial screening test, usually an immunoassay. This test uses a relatively higher ng/ml threshold to quickly separate negative samples from those requiring further analysis. Any sample with a concentration below this initial cutoff is immediately reported as negative and requires no further testing.
If the concentration of the drug or metabolite exceeds the initial ng/ml cutoff, the sample is classified as “presumptive positive” and proceeds to the second step. This confirmation analysis uses precise and sophisticated technology, such as Gas Chromatography/Mass Spectrometry (GC/MS) or Liquid Chromatography/Mass Spectrometry (LC/MS). The confirmation test is capable of identifying and quantifying the exact drug metabolite with high accuracy, and it applies a lower, more specific ng/ml cutoff.
For instance, a urine sample that screened positive for marijuana at 50 ng/ml would be subjected to the confirmation test. This test uses a lower cutoff, typically 15 ng/ml, to verify the presence of the specific metabolite, 11-nor-9-carboxy-THC. The final result is reported as positive only if the concentration of the specific metabolite exceeds this lower confirmation ng/ml threshold. If the sample’s concentration falls between the initial (50 ng/ml) and the confirmation (15 ng/ml) cutoffs, or if the specific metabolite is not confirmed, the final result is reported as negative.