Windows of Detection

The window of detection is the time that a drug can be detected in a biological sample above a specified cut-off for the test being performed. There is a period of time immediately before and after this detection window when the drug will be present in the sample but at a level below the cut-off.

Tests are designed to be drug or drug group specific and the cut-off level has been determined to optimise detection without giving false positives.

Drugs are detected in oral fluid either from direct deposition in the mouth or by transfer from the blood stream following ingestion and absorption. Any drugs present in the bloodstream are metabolised in the liver before being excreted in the urine. This process results in drugs appearing in the urine later than oral fluid. The drugs are detectable for a significantly longer time in urine than in oral fluid.

Figure 1: A visual representation of a cut-off level and window of detection.

The actual time a drug will remain detectable in a sample will depend on some or all of the following:

• The amount of drug taken
• How frequently the drug is taken
• The nature of the drug itself
• An individual’s metabolism and general health
• The amount of fluids taken since taking the drug
• The amount of exercise taken since taking the drug
• Genetic variations that affect a response to drugs

For example if a person smoked a single spliff the cannabis could remain detectable in urine for no more than 2–3 days and may even be as short as one day depending on the strength of cannabis. However if their cannabis use is habitual and heavy it is stored in the fatty tissues, resulting in a much wider detection window (sometimes up to 30 days).

The data supplied below acts as a guideline and should be interpreted very carefully due to the large number of factors that can influence the level of drug over time.

Oral Fluid

The shortest window of detection is found with oral fluid.

Urine

Drugs can be detected in urine for longer, and in consequence the window of detection for some drugs can be affected by whether their effects are short or long acting.

Hair

Hair provides a historic record of drug use, and detection windows are based entirely on hair length. It takes about 14 days for the drugs to appear in the hair shaft, and moving away from the scalp, every 1cm of hair length approximates to a one month window of detection.