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National Veterinary Services Laboratories Scrapie/Chronic 

Wasting Disease (CWD) Immunohistochemistry (IHC) 

Proficiency Test Summary

1. Composition of proficiency test panel (PT): The scrapie/CWD IHC panel 

consists of a minimum 5 unstained histologic sections (formalin-fixed, paraffin 

embedded) of sheep/deer/elk - brains/lymph nodes on glass slides.

2. Cost of proficiency test: There is no charge to NAHLN approved laboratories. 

Currently there is no charge to other laboratories (subject to change). Click here to 

view current NVSL Catalog of Reagents. Additional information on diagnostic 

reagents, including ordering, payments, shipping details and fees, is located at 

www.aphis.usda.gov/nvsl. Then click on the Reagents and Proficiency Tests link.

3. Storage conditions: Samples are stored at room temperature (18°-26°C).

4. Sample preparation/selection criteria: Samples selected are from various samples 

submitted from field suspect cases of scrapie/CWD. These selected samples are tested by 

IHC and vary from positive to weakly positive to negative for the presence of prion 

protein.

5. Panel quality control: All slides are cut at one time. The NVSL stains the first and 

last slide of the series for consistency and evaluates the staining quality and staining 

morphology.

6. Timing of the proficiency test distribution and data collection: The PT is issued 

annually with a target of the 4th FY quarter (July-Sept). 

7. Test method: The PT samples are tested by IHC staining of panel slides using 

approved methods. Subsequent evaluation and scoring/interpretation of the resultant 

staining are done by the PT participants, and the slides and results are then sent to NVSL 

(Head of Pathology, Parasitology, and Entomology and/or designee) for final evaluation.

8. Submitting test results: Participants are required to have the results of staining 

interpretations and the stained slides submitted to NVSL not later than 10 days after the

panel has been received by the participant.

9. Scoring of individual panel samples: For each panel a score is based on number of 

slides with correct interpretation (Positive/Not Detected) by the participant. Additional 

dialog or training may be required if interpretations do not include the identification of 

inappropriate tissues locations, or if incorrect diagnostic terminology is used in reporting 

results.

10. Laboratory pass/fail criteria: A final score of 100 percent correctly identified 

slides (Positive and Not Detected) is considered as a passing score.
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11. Reporting laboratory test scores: Panel results and slides are sent to the 

appropriate NVSL personnel, who will share by (email/postal mail), the results with 

each individual participant (Laboratory Director or designee). Results will be shared 

within 3 weeks after receipt of the results and slides from all panels.

12. Remedial actions required for failing laboratories: If discrepant results are 

obtained due to misinterpretation, additional training may be required. The NVSL may 

issue a larger pre-stained panel for interpretation and scoring. If discrepant results in the 

interpretation persist this may be considered grounds for withdrawal of laboratory 

approval.

If discrepant results are caused by suboptimal staining (or clearly suboptimal staining is 

observed) NVSL will work with the laboratory or other sources to determine the cause of 

the suboptimal staining. If it is determined to be laboratory related and suboptimal 

staining persists this may be consider grounds for withdrawal of laboratory approval.

13. Special requirements: None.