Laboratory Accreditation in Microbiology: ISO 15189, NABL, and What It Means

A clinician trusts a culture result from a laboratory without ever seeing how that laboratory works, whether the staff are competent, whether the methods are sound, whether the equipment was working. That trust does not come from the report itself; it comes from a mark on it saying an independent body has examined the laboratory and confirmed it is competent to produce that result.

That examination is accreditation, and understanding what it actually certifies, and what it does not, is part of understanding how laboratory results earn the confidence placed in them.

First, where accreditation sits

Quality in a laboratory is built in layers. Quality control (QC) checks that a test is working now; quality assurance (QA) is the wider system that makes the whole process reliable; and the quality management system (QMS) is the documented framework that organizes it all. How those three nest, and the day-to-day QC and quality-assessment activities inside them, are covered in the quality control in the microbiology laboratory overview.

Accreditation sits on top of that structure. It is the point at which an outside body examines the laboratory's whole quality system and its technical work and formally confirms both are sound. This article is about that top layer: what accreditation is, what standard it is measured against, who grants it, and how a laboratory earns and keeps it.

What accreditation is

Accreditation is formal recognition by an independent, authoritative body that a laboratory is competent to carry out specific tests to a defined standard. Two words in that sentence carry the weight. Independent: the judgment comes from outside the laboratory, not from its own staff. Competent: the recognition is not just that the laboratory has procedures, but that it can actually produce correct results.

Accreditation is always scoped to specific tests. A laboratory is not accredited in general; it is accredited for the particular tests or disciplines it has been assessed for. A microbiology laboratory might be accredited for its bacterial culture and susceptibility testing but not for a molecular assay it has only just introduced. The scope of accreditation is part of what the certificate states.

Accreditation versus certification: the distinction that matters

These two are constantly confused, and the difference is easy to state but easy to get wrong.

• Certification is confirmation that a laboratory has a quality management system that conforms to a standard. It answers: does the machinery of quality exist and is it documented? A laboratory can be certified to a general quality standard (such as ISO 9001) by showing it has the processes and follows them.
• Accreditation goes one step further and confirms technical competence as well as conformance. It answers not only "does the quality system exist?" but "can this laboratory actually produce correct, reliable results for these specific tests?" That second question is assessed by technical experts who examine the methods, the equipment, the personnel's competence, and the results themselves, not just the paperwork.

For a medical laboratory, competence is the point. A well-documented quality system that still produces wrong culture results helps no patient. This is why medical laboratories pursue accreditation, which tests competence, rather than certification alone, which tests conformance. The short form: certification says the system exists; accreditation says the laboratory is competent.

ISO 15189: the standard for medical laboratories

Accreditation is measured against a standard, and for medical laboratories that standard is ISO 15189, "Medical laboratories, requirements for quality and competence." The title states its two halves, and both matter.

1. The quality (management) requirements cover the organizational side: the quality management system, document control, management of records, handling of errors and complaints, internal audit, and continual improvement, the same QMS structure the QC overview describes.
2. The competence (technical) requirements cover the ability to produce correct results: the competence and training of personnel, the suitability and maintenance of equipment, the validation and verification of methods, the control of the pre-analytical and post-analytical phases, and the accuracy of reporting.

This second half is what distinguishes ISO 15189 from ISO 9001. ISO 9001 is a general quality-management standard usable by any organization; it checks that a quality system conforms. ISO 15189 adds the technical competence requirements specific to medical testing, which is why it, and not ISO 9001, is the standard on which medical-laboratory accreditation is built. A laboratory can hold ISO 9001 certification and still not be competent to release patient results; ISO 15189 accreditation is designed to close exactly that gap.

Who grants accreditation: the accreditation bodies

Accreditation is granted by a national accreditation body, an organization authorized to assess laboratories against ISO 15189 and confer accreditation. In India, this is NABL, the National Accreditation Board for Testing and Calibration Laboratories. Other countries have their own bodies (for example, CAP and A2LA in the United States, UKAS in the United Kingdom), and international arrangements allow accreditation by one recognized body to be accepted across borders.

Whatever the body, its role is the same: to send trained assessors, including technical experts in the relevant discipline, to examine the laboratory against the standard and decide whether it meets the requirements for the tests in its scope. The body does not run the laboratory or improve it; it independently judges it, which is what makes its recognition credible to the clinicians and patients who rely on the results.

How a laboratory achieves accreditation

Accreditation is earned through a process, not bought. The path is broadly the same across bodies.

The laboratory first builds and runs its quality management system: it writes and controls its SOPs, establishes QC and quality-assessment activities, trains and assesses staff, maintains and calibrates equipment, and generates records showing all of this is happening. Crucially, it must run this system for a period before assessment, because accreditation examines evidence of sustained practice, not a snapshot.

The laboratory then applies to the accreditation body and undergoes assessment. Assessors review the documentation (the quality manual, SOPs, records), then visit the laboratory to observe practice directly, checking that staff actually follow the SOPs, that QC and EQA records are complete, that equipment is maintained, and that the laboratory does what its documents say. Where the assessors find gaps (non-conformities), the laboratory must correct them and show evidence of correction before accreditation is granted.

Accreditation is then awarded for the specific scope of tests assessed. It is common for the assessment to reveal issues that must be resolved first, so preparing for accreditation is itself a substantial quality-improvement exercise, often the point at which a laboratory's quality system genuinely matures.

How accreditation is maintained

Accreditation is not a permanent certificate. It is granted for a defined period and must be maintained and renewed.

Between assessments, the laboratory must keep operating its quality system and demonstrate ongoing compliance: continuing QC and EQA, internal audits, management review, and correction of any problems that arise. The accreditation body conducts periodic surveillance assessments during the accreditation cycle and a full reassessment before renewal. If a laboratory stops meeting the requirements, its accreditation can be suspended or withdrawn for the affected scope.

This ongoing nature is the point. Accreditation certifies sustained competence, not a single passing inspection, which is why it carries the weight it does. A one-time pass would prove little; continued accreditation shows the laboratory holds the standard over time.

Why accreditation matters for the laboratory and the patient

Accreditation delivers value on several levels. For the patient and clinician, it provides independent assurance that results can be trusted, which is the whole purpose. For the laboratory, the discipline of achieving and holding accreditation drives real improvement: better documentation, tighter QC, competent staff, maintained equipment, and fewer errors. It also enables recognition of results across laboratories and borders, and it is increasingly required for reimbursement, for participation in networks, and for institutional credibility.

The deeper point is that accreditation is the external expression of everything else in the quality structure. A laboratory cannot be accredited without working SOPs, functioning QC, competent staff, and a genuine QMS. Accreditation is not a separate activity bolted on at the end; it is the independent confirmation that the whole quality system, the subject of the QC overview and the SOPs that run it, is real and working.

How to remember

• Certification says the system exists; accreditation says the lab is competent. The one distinction to hold above all others. Accreditation checks technical competence, not just that procedures are documented.
• ISO 15189 = quality + competence, which is why it beats ISO 9001 for medical labs. ISO 9001 checks the system conforms; ISO 15189 adds the technical competence requirements specific to medical testing.
• Accreditation is scoped, earned, and ongoing. It applies to specific tests, is granted only after assessors verify sustained practice, and is maintained through surveillance and reassessment. Not general, not bought, not permanent.
• It rests on everything below it. No working SOPs, QC, competent staff, and QMS, no accreditation. Accreditation is the external confirmation that the quality structure is genuinely working.

Key facts

Where students get confused

"Accreditation and certification are the same thing." Certification confirms a quality system conforms to a standard. Accreditation confirms both conformance and technical competence to produce correct results. For medical laboratories, accreditation (against ISO 15189) is the meaningful goal, because competence, not just documented procedures, is what protects the patient.

"ISO 15189 and ISO 9001 are interchangeable." ISO 9001 is a general quality-management standard that checks a system conforms. ISO 15189 is specific to medical laboratories and adds technical competence requirements (personnel competence, method validation, equipment, reporting) that ISO 9001 does not. Medical-lab accreditation uses ISO 15189.

"A laboratory is accredited for everything it does." No. Accreditation is scoped to the specific tests or disciplines assessed. A laboratory can be accredited for some tests and not others, and a newly introduced test is not covered until it has been assessed and added to the scope.

"Once accredited, always accredited." Accreditation is granted for a period and maintained through ongoing compliance, surveillance assessments, and reassessment. It can be suspended or withdrawn if the laboratory stops meeting the standard. It certifies sustained competence, not a one-time pass.

"Accreditation is paperwork you complete at the end." It is the independent confirmation that the entire quality system, SOPs, QC, competent staff, maintained equipment, is genuinely working. A laboratory cannot be accredited without those already in place and running; preparing for accreditation is what often matures a laboratory's quality system in the first place.

References

1. International Organization for Standardization. ISO 15189: Medical laboratories, Requirements for quality and competence. Geneva: ISO.
2. Carey RB, Bhattacharyya S, Kehl SC, et al. Implementing a Quality Management System in the Medical Microbiology Laboratory. Clin Microbiol Rev. 2018;31(3):e00062-17. doi:10.1128/CMR.00062-17
3. World Health Organization. Laboratory Quality Management System: Handbook. Geneva: World Health Organization.
4. Leber AL, editor. Clinical Microbiology Procedures Handbook. 4th ed. Washington, DC: ASM Press; 2016. doi:10.1128/9781683670438.CMPH