Root cause analysis

You asked firstly

what is your advice to avoid this problem

As you mentioned a delay in receiving a report of approx 3 months, I shall reply, assuming you are referring to the report you receive from a proficiency testing (PT) scheme.

The purpose of a management system is to ensure that all the supporting activities are effective and any risks are under control (quality assurance). Ensuring the validity of your results, meaning that the results are valid and can be released to customers; involves a dual approach of internal and external quality control.  Now the first issue at hand is to ask  - is it appropriate to only be evaluating your external performance every three months ? Look at the risk of releasing invalid results in the time between PT reports. If your objectives are impacted by only receiving PT reports every three months, you need to address the risk. It will depend on the method – for example if you have performed validation using certified reference materials, or even run a CRM with every batch (i.e. internal quality check) you are assured of the accuracy and have established you do not have a significant bias.  You would release the routine results, based on the internal quality control checks passing for the same batch as the routine samples as well as having no trends in your PT scheme performance over the prior period (i.e. no changed risk). If there was a poor performance previously, your assurance would be that the laboratory had already implemented effective, verified corrective action.

You also asked  

How to make the root cause investigation easy?

To address your question, it is important to ask the purpose of root cause analysis. In the context of corrective action, it is a process used by a laboratory during evaluation of a nonconforming event, to determine the basic underlying reason for the deviation from a desired outcome. Before starting, look at  the risk that the labortory's stated desired outcome stated or specified criteria was not appropriate.

So what do you need to consider, with when doing performance evaluation ? Remember that PT is a way to confirm that over a period of time (because it has a statistical basis) your laboratory does not have a significant bias or trend. Make sure that the pre-defined performance criteria you set for each method is appropriate. To avoid unnecessary investigation these must be realistic. For example it will not be appropriate to state that for all events where a PT report indicates a single unsatisfactory result, the laboratory would do corrective action. You need to evaluate the significance of the reported PT performance.  State the reported deviation from the pre-defined performance criteria and evaluate, according to ISO 17025 clause 7.10 whether corrective action is required. For example the assigned value may have been below the Limit of quantification for your method or not a suitable matrix. meaning ther is no reason for corrective action.

The following ISO 17025 document template: may assist further:

Quality Assurance Procedure at https://advisera.com/17025academy/documentation/quality-assurance-procedure/

Complaint, Nonconformity and Corrective Action Procedure at https://advisera.com/17025academy/documentation/complaint-nonconformity-and-corrective-action-procedure/

For more information on root cause, have a look at the article How to use root cause analysis to support corrective actions in your QMS at https://advisera.com/9001academy/blog/2016/03/01/how-to-use-root-cause-analysis-to-support-corrective-actions-in-your-qms/