Why “latent errors” is not a good term

One occasionally hears the term “latent errors” in articles about error reduction techniques (1). The purpose of this essay is to explain problems with this term and to suggest alternatives.

Latent implies hidden. Berwick uses the term “latent failures” and equates this with “the little things that go wrong all the time”. These are misleading concepts. Consider an example. In a recent presentation, Astion presents some examples of latent errors and their effects (2).

  • Computers: A lack of 1 instrument interface is responsible for many active data entry errors.
  • Staffing: 1 latent error regarding suboptimal staffing leads to multiple active errors by staff who are forced to multitask.
  • Policy and Procedure: A bad strategy for handling phone calls can lead to multiple errors

Before analyzing one of these examples, consider a fault tree model of lab error. This is a hierarchical (“top down”) chart of errors which has the following properties:

  • The severe errors are at the top
  • Errors are connected through parent – child connections
  • The parent errors are the “effects” of the child errors
  • The child errors are the causes of the parent errors
  • The tree uses “gates” which include:

o       or gate means any child event in that branch that occurs will cause the parent error

o       and gate means all child events in that branch must occur to cause the parent error

o       basic gate is the end (cause) of a tree branch

  • Each error is classified as to its:

o       severity

o       probability (likelihood of occurrence)

Considering the staffing error mentioned by Astion. One could postulate one of many branches of a fault tree to contain this error as:

Top – outlier (e.g., incorrect result) reported to clinician

AND – assay has interference to lipemia

AND – sample is lipemic

AND – (visual) detection for lipemic sample failed

OR – technician not available to perform test

OR – technician called away

BASIC – inadequate staffing

Translating the events of this tree into English (sort of), an outlier will be reported if the assay has an interference to lipemia, AND the sample is too lipemic AND the step for visually examining the sample has failed. The visual examination step failure can have several causes, one of which is the technician does not perform this step. This has several causes, one of which is the technician has been called away because the staffing is inadequate (e.g., there is a problem somewhere else that should be handled by staff but, inadequate staffing prevents this).


An outlier that is reported to a clinician is among the most severe errors that a lab can make. Every event in this branch of the tree has the same severity classification because the effect of any of these errors is the top level error. The importance (ranking) of these errors may be different because the probability of each of these errors may be different.

Because the severity is high, there is no reason for calling any of these events “the little things that go wrong.” They are all severe events. There is also no reason to call them latent (e.g., hidden) or to call the top level error “active” which implies that the lower level errors are not active. Any of these events that occurs are active. Whether these events can be detected depends on programs in place (3) to expose such errors.

To summarize what FMEA recommends:

  1. Flowchart the process
  2. Add process steps to a fault tree
  3. Add causes to each potential process step error
  4. Add FMEA information to each event
  5. Rank the errors
  6. Propose mitigations


Before deciding that one has thought of all possible causes for an error, one should consult a list of “QSEs (Quality System Essentials) (3). These are generic activities that apply to virtually all aspects of a service. These activities will typically not appear in flowcharts because they are so pervasive that they would make flowcharts too complicated.

For an additional critique of reference 1, see reference 4.


  1. Berwick, DM. Errors Today and Errors Tomorrow N Engl J Med 2003;348: 2570-2572
  2. Astion M. Developing a Patient Safety Culture in the Clinical Laboratory http://www.aacc.org/AACC/events/expert_access/2005/saftey/
  3. Application of a Quality System Model for Laboratory Services; Approved Guideline—Third Edition GP26-A3 NCCLS 2004 Wayne, PA
  4. Krouwer JS There is nothing wrong with the concept of a root cause. Int J Qual Health Care 2004;16:263

See also additional references at the end of the systems not people essay.


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