From the Guidelines
Urine samples for drug screening should be refrigerated and tested within 8 hours for optimal accuracy, as longer storage times may lead to microbial growth, cell debris, and degradation of labile biomolecules. According to the most recent and highest quality study 1, storage time is a critical factor in maintaining the integrity of urine samples. The study recommends that samples should be stored at a maximum of 4°C and processed within 8 hours to minimize degradation.
When refrigerating urine samples, it is essential to store them in a sealed container at 2-8°C (36-46°F) to prevent contamination or evaporation. Some key considerations for handling refrigerated samples include:
- Allowing them to return to room temperature before testing
- Mixing thoroughly to ensure accurate results
- Using a clean, leak-proof container that is resistant to urine pH-range and does not shed plastic particles
- Recording the material, manufacturer, and lot number of the container
The refrigeration process helps preserve most drugs and their metabolites by slowing bacterial growth and enzymatic processes that could otherwise break down these compounds or produce false results. For chain-of-custody testing or legal purposes, it is crucial to follow specific laboratory protocols, as requirements may be more stringent than for clinical testing. In contrast to older studies, such as 1, which suggests refrigerating urine samples for assay the same or next day, the more recent study 1 provides a clearer guideline for optimal storage time.
From the Research
Urine Drug Screen Storage
- The stability of drugs of abuse in urine samples is crucial for accurate testing results 2
- Refrigeration is recommended for storing urine samples, but the duration of storage is not specified in the provided studies
- A study on the stability of common analytes in urine refrigerated for 24 hours found that the stability varies with the substance tested 3
- Some substances, such as leukocyte esterase and hemoglobin, may degrade after 24 hours of refrigeration, while others, such as nitrites, glucose, and ketones, remain stable 3
Storage Conditions
- Refrigeration at 4 degrees C is recommended for storing urine samples to prevent bacterial overgrowth 4
- Centrifugation to remove cell debris and the use of preservatives, such as sodium azide or boric acid, can also help prevent bacterial overgrowth 4
- The addition of a salt mixture can help slow the degradation of some drugs of abuse in urine samples kept at room temperature 2
Testing Methods
- Gas chromatography/mass spectrometry (GC/MS) is a common method for comprehensive urine drug screening 5
- Immunoassays are also widely used for urine drug screening, but may produce false-positive or false-negative results 6
- Secondary analysis, such as GC/MS, is often required to confirm the results of immunoassays 6