> For the complete documentation index, see [llms.txt](https://docs.snomed.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.snomed.org/snomed-ct-specifications/snomed-ct-release-file-specification/snomed-ct-identifiers/6.4-check-digit.md). # Check-digit The final digit of the [SCTID](/snomed-ct-specifications/snomed-ct-release-file-specification/appendices/appendix-b.-specification-reference-information/s/sctid-data-type.md) is a check-digit. Users should be required to type SCTID values but in some case during design and development it may be necessary to copy or paste identifiers. The objective of the check-digit is to detect the commonest types of error that may occur due to typographical errors on those situations or in other cases where transcription or communication mechanisms may introduce error. Examples may include high-level development such as creating or modifying protocols or pre-specified queries. An SCTID is checked by using the Verhoeff check, which is a Dihedral D 5 Check. This detects a higher proportion of common typographical errors than either the IBM or Modulus 11 check. Unlike the Modulus 11 check it is effective on decimal strings longer than ten-digits. Furthermore its value can always be represented as a decimal digit without excluding any values. ## Related Links * See [Check-Digit Computation ](/snomed-ct-specifications/snomed-ct-release-file-specification/snomed-ct-identifiers/6.4-check-digit/6.4.2-check-digit-computation.md)for detailed information about the Verhoeff check-digit algorithm and links to sample program code. * A live site to check a SNOMED CT Identifier can be found here - Provide Feedback --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://docs.snomed.org/snomed-ct-specifications/snomed-ct-release-file-specification/snomed-ct-identifiers/6.4-check-digit.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.