At-Home Cannabis Test Kit: Detection Times & Accuracy

At-home cannabis test kits detect THC or THC metabolites in urine, saliva, or hair. Most kits use urine. Detection time depends on how often you use, body fat, and metabolism. Urine: 3 days after one use, up to 30 days after daily use. Saliva: 1 to 24 hours. Hair: up to 90 days.

What the kits test for

Urine kits detect THC-COOH, a metabolite. Saliva kits detect THC itself. Hair kits detect THC-COOH. Most kits use immunoassay. A chemical reaction changes a line color on a strip.

Detection windows

• Urine: 3 days after one use. 5 to 10 days after moderate use. 15 to 30 days after chronic use.
• Saliva: 1 to 24 hours. Up to 72 hours after heavy use.
• Hair: up to 90 days. Hair grows 0.5 inch per month.

Cutoff levels

The federal workplace cutoff for urine THC-COOH is 50 ng/mL. A confirmatory test uses 15 ng/mL. Some home kits use 50 ng/mL. Others use 20 ng/mL. A lower cutoff finds more positives and stays positive longer.

Accuracy

Home kits have 95% to 99% sensitivity for THC at 50 ng/mL. False positives come from NSAIDs, efavirenz, and some proton pump inhibitors. False negatives come from diluted urine. Drinking 1 quart of water before a test can drop metabolite levels below cutoff.

How to use a urine kit

1. Collect urine in a clean cup.
2. Dip the strip for 10 to 15 seconds.
3. Wait 5 minutes.
4. Read the lines. Two lines: negative. One line: positive. No control line: invalid.

Cost and limits

Kits cost $10 to $50. Sold online, at pharmacies, and at some cannabis stores. No at-home kit measures impairment. No kit gives a THC blood level. No at-home blood test exists. A lab test with GC-MS or LC-MS is needed for confirmation.