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Clean Technique

Home > Forensic Biology > Laboratory Orientation > Clean Technique

Clean techniques refer to laboratory practices employed to reduce the risk of contamination. Clean techniques are employed in the forensic DNA laboratory to prevent the transfer of DNA from analyst to sample, environment to sample, and cross-contamination between samples. Contamination can adversely effect the outcome of a case; therefore, it is essential that the laboratory have procedures in place to limit, recognize, and address contamination.

Effective clean techniques procedures assist the laboratory in meeting the QAS 6.1.4, which requires that laboratories have and follow a written procedure for monitoring, cleaning, and decontaminating facilities and equipment. Specific techniques are not delineated by the standards. It is the responsibility of laboratory management to design and implement appropriate clean techniques protocols. Some recommended practices are provided.

Recommended Practices for Clean Technique

Work Surfaces and Equipment

  • Work surfaces should be cleaned before contact with evidence, between evidence items, and after evidence processing is complete.
  • It is common practice for glassine weigh paper, Kimwipes®, butcher paper, or Benchkote® paper to be placed on the bench top while processing evidence to act as a barrier. The paper should be changed and the bench top cleaned between items.
  • Centrifuges, thermal cycler, tube racks, pipettes, and any other equipment deemed appropriate should be cleaned before and after each use.
  • Instruments such as forceps, scissors, scalpels, and tube openers should be cleaned just prior to use. Some laboratories purchase sterile disposable instruments. These should be opened just prior to sample processing and discarded after one use.
  • Cleaning should be done with a 10% bleach solution or a commercially available sterilization reagent such as Cidex® Plus.
  • After an item or surface is cleaned with bleach it must be rinsed with purified water or alcohol to prevent the build up of sodium hypochlorite crystals. Instruments or equipment cleaned with bleach should be rinsed to avoid corrosion.

Reagents and Sample Processing

  • When appropriate, reagents should be prepared in bulk. Each analyst is then provided with an aliquot for his/her individual use.
  • Reagents should be kept closed when not in use.
  • Samples should be processed individually. Only one sample should be open at a time.
  • Unknown samples should be processed separately from reference samples. Processing may be separated by time and/or space. When possible, small/dilute samples should be worked prior to large/concentrated samples.
  • Autoclave sample tubes.
  • Only one microcentrifuge tube should be open at a time. Close each tube immediately after labeling and after the addition of sample or reagents to prevent cross-contamination.
  • Use a tube opener, clean Kimwipe®, or other suitable barrier, rather than gloved fingers, to open microcentrifuge tubes.
  • Aerosol-resistant pipette tips should be used. Place the sterile tip on the pipette immediately prior to use. If the pipette is set down with the tip on, discard the tip. A new pipette tip should be used for the addition of each reagent to a sample tube.
  • Centrifuge microcentrifuge tubes prior to opening to remove any liquid clinging to the lid.
  • Use basket tubes, such as Spin-X® tubes, to centrifuge stain extraction buffers from sample matrices.

Good Lab Practices

  • Gloves should be worn throughout sample processing. At a minimum, gloves should be changed at the completion of each step of the process. If gloves become contaminated, discard them and replace with new ones.
  • Lab coats should be worn at all times while processing evidence. It is essential for a lab coat and gloves to be worn at all times in the post-amplification room.
  • The post-amplification room should contain lab coats, gloves, and equipment that do not leave that area without decontamination. Adherence to this practice prevents contamination of pre-amplification areas with amplified product.
  • The movement of paperwork from post-amplification into pre-amplification areas should be limited . The most common solution is to send data to printers outside the laboratory.

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© 2007 NFSTC Science Serving Justice®
NOTE TO USERS: The President’s DNA Initiative DNA Analyst Training program and assessment were completed and published in 2005, in cooperation with the National Institute of Justice. The science and techniques in the program are sound and proven, however, program content has not been updated to include tools and technologies developed and in use after 2005, including many kits and robots. Assessment questions address only content delivered in this program and may not contain the full range of tools in use in your laboratory.