American Association for Clinical Chemistry
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Patient Safety Focus: Playing It Safe by Having 'Defense in Depth' in Place

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 Playing It Safe by Having 'Defense in Depth' in Place
By Karen Appold and Michael Astion, MD

Firewalls, video monitors, and number-coded locks: all of these are security measures. But all are vulnerable to threats. Is there any way to ensure that these systems will never fail? One strategy the military uses is called defense in depth. It involves putting in place multiple layers of security, just in case the first line of protection fails.

Very familiar examples of defense in depth can be found in everyday life. Take safety deposit boxes at banks. Your safety deposit box doesn’t merely have a lock on it. There is also a lock on the door to the safety deposit box room, and a lock on the doors of the bank itself (1, 2).

Defense-in-depth strategies are commonplace.
The bank’s safety deposit boxes are protected
by a massive locking door, as well as
the locks on the doors of the bank.

The concept of defense in depth can also be applied to processing specimens in the clinical lab. For example, labs usually employ multiple strategies for protecting against the threat of mislabeled specimens. The most significant threat of a mislabeled specimen is that a patient will receive the wrong result, one which is from another patient. If the caregiver acts on this erroneous result, it can be a potentially serious error, causing harm to the patient.

Defense in depth against the threat of mislabeling begins by requiring two or more identifiers for every specimen rather than one. Another defense is having the phlebotomist ask the patient to actively provide one or more of these identifiers. Staff within the lab provides the next layer of defense by verifying that the two identifiers on the specimen match the two identifiers on the test requisition. If the identifiers do not match, the specimen gets rejected.

The next line of defense in depth comes from delta checks used on automated instruments. These automated methods compare the difference or delta between today’s test value and the corresponding previous test values with a given threshold. Delta check failures often reveal mislabeled specimens.

The direct care provider, usually a physician or nurse, also provides a line of defense. Caregivers often become suspicious when lab results do not match their clinical impression. This situation usually leads to redrawing the patient’s specimen, thereby revealing the erroneous result caused by the mislabeled specimen. A final line of defense is the patient, who may block an erroneous result caused by mislabeling, when he or she suspects the result is incorrect and asks for a redraw.

Some labs also have multiple layers of defense to prevent data entry errors. One approach involves requiring two lab professionals to separately log the information from paper requisitions into the lab information system (LIS). Only if the two independent sets of data entry agree is the order allowed to move forward. Another layer of defense is to have customer service center personnel check the next day to see that the LIS order matches the order on the paper requisition.

Figure 1
Five Layers of Defense Against the Threat of Mislabeling

Each vertical line represents a layer of defense to protect patients from the harm of mislabeled specimens.

Preventing against mislabeled specimens is the most obvious process to employ a defense-in-depth strategy in the lab, but other processes in the lab can also benefit from its application. Laboratory leadership often employs defense in depth strategies to defend against the threat of instrument downtime. One strategy is to have two or more identical instruments, with identical menus, on an automated line. Another strategy is to have a plan to send specimens to another lab in case of a lab disaster.

Finally, the concept of defense in depth can also be applied to lab staffing. Labs should have multiple strategies for ensuring that they have sufficient staff. This includes: 1) operating the lab with excess capacity (i.e., operating the lab at 70–80% of capacity rather than 100% in order to have extra capacity in case of emergencies); 2) having staff on-call who can rapidly come to work; 3) employing temps as backup staff; and 4) having plans for helping staff get to work in case of weather emergencies, as well as disasters.

Together, these multiple lines of defense can help lab strengthen your lab’s safety.


  1. The Old New Thing. Available at: Accessed April 25, 2010.
  2. Defense in Depth. Available at: Accessed April 25, 2010.

Sponsored by ARUP Laboratories, Inc.