Standard Operating Procedures (SOPs) in Microbiology: What They Are and How to Write One

Two technologists run the same test on the same sample and get two different results. Neither made an obvious mistake; they simply did small things differently, incubated a little longer, read the endpoint slightly differently, used a different control.

In a laboratory, that variation is unacceptable, because a patient's treatment depends on the result being the same no matter who is on shift. The document that removes that variation, that makes the test happen the same way every time, is the standard operating procedure. Learning to read, follow, and write one is one of the first professional skills a microbiologist needs, and one of the least taught.

What a standard operating procedure is

A standard operating procedure (SOP) is a written, approved document that describes exactly how to carry out a specific laboratory activity, step by step, so that anyone trained to use it performs the activity the same way every time. It turns "how we do this test" from something held in an experienced person's head into a written standard the whole laboratory follows.

SOPs cover far more than tests. A laboratory has SOPs for technical procedures (how to perform a Gram stain, set up a culture, run a susceptibility test), for equipment (how to operate, calibrate, and maintain the autoclave or the biosafety cabinet), for quality control (how and when to run control strains), for safety (how to handle a spill, how to dispose of waste), and for the steps around testing (how to accept or reject a specimen, how to report a result). If an activity affects the quality or safety of the laboratory's work, it should have an SOP.

Why the laboratory runs on SOPs

SOPs exist to solve a specific, serious problem: variation. The reasons they matter follow directly from that.

1. They make results consistent and reproducible. When every technologist follows the same written steps, the same sample gives the same result regardless of who runs it or when. Without an SOP, each person's individual habits become a hidden source of variation, and in a diagnostic laboratory that variation can change a patient's diagnosis.
2. They preserve knowledge and train new staff. An SOP captures how an experienced microbiologist actually does a procedure, so that knowledge does not walk out the door when that person leaves or goes on leave. A new staff member can be trained against the SOP and assessed on whether they follow it.
3. They are the foundation of quality and accreditation. Every quality system and every accreditation standard (such as ISO 15189) is built on documented procedures. An accredited laboratory must have current, approved SOPs for its activities and must be able to show that staff follow them. SOPs are not paperwork added for inspection; they are the written form of the laboratory's quality.
4. They support safety and accountability. A safety SOP tells staff exactly what to do in a risky situation (a spill, an exposure, a fire) before they have to think about it under pressure. And because an SOP is dated, versioned, and approved, it creates a clear record of what the laboratory's agreed practice was at any point in time.

The anatomy of an SOP: the sections it must contain

A good SOP has a predictable structure, so that any reader knows where to find what they need. The exact headings vary between laboratories, but a complete SOP contains these elements.

1. Header and document control. Every SOP carries a title, a unique document number, a version number, the effective date, and the pages (e.g. "page 2 of 6"). It records who wrote it, who reviewed it, and who approved it, with dates and signatures. This document-control block is what makes an SOP traceable and is the first thing an assessor checks.
2. Purpose (scope). One or two sentences stating what the procedure is for and when it applies. For example, "This SOP describes the Gram staining of bacterial smears from culture and clinical specimens." The scope prevents the SOP being used for something it was not written for.
3. Responsibility. Who is authorized and competent to perform the procedure, and who is responsible for supervising or approving it.
4. Materials and equipment. Everything needed to perform the procedure: reagents (with concentrations), media, consumables, and instruments. Listing them means the procedure can be set up without guesswork.
5. Safety considerations. The specific hazards of this procedure and the precautions for them, personal protective equipment, biosafety level, what to do with waste. Safety is not assumed; it is stated for this procedure.
6. Procedure (the step-by-step method). The core of the SOP: the actual steps, in order, numbered, each one an unambiguous instruction. This is where most of the writing skill goes, and the next section is about how to write it well.
7. Quality control. How the procedure's performance is checked: which control organisms or materials to run, how often, what the expected results are, and what to do if the control fails. A technical SOP without a QC section is incomplete, because it does not say how to know the procedure is working.
8. Interpretation and reporting. How to read the result and how to record or report it, including any critical results that need immediate communication.
9. References. The sources the SOP is based on (a standards document, a manufacturer's instructions, a textbook), so the procedure can be traced to its authority.

How to write the procedure steps well

The step-by-step method is where an SOP succeeds or fails as a usable document. A few principles separate a clear SOP from one that sits in a folder unread.

1. Write for the person who will actually do it. The SOP should let a trained but not expert person perform the procedure correctly by following it. That means enough detail that nothing critical is left to assumption, but not so much that the reader is buried. Aim for the level of a competent new staff member, not an expert who already knows the steps and not a complete novice who needs a textbook.
2. One action per step, in the order it is done. Break the procedure into numbered steps, each a single clear action. "Incubate the plate at 35 to 37 °C for 18 to 24 hours" is one step. Bundling several actions into one paragraph is how steps get skipped.
3. Be specific and unambiguous. State the actual numbers and conditions, not vague ones. "Incubate overnight" is ambiguous; "incubate at 35 ± 2 °C for 18 to 24 hours" is not. "Add a small amount of reagent" is ambiguous; "add two drops of reagent" is not. Every place a reader could reasonably do two different things is a place the SOP has failed to standardize.
4. Use plain, direct language and the active voice. Write "Flood the smear with crystal violet," not "The smear should be flooded with crystal violet." Short, direct instructions are followed correctly more often than long ones, and this matters even more where staff read in a second language.
5. Include the critical details that change the result. Temperatures, times, concentrations, volumes, the order of steps, and the endpoints (how to know a step is complete or a reaction is positive) are the details that determine whether two people get the same answer. These are exactly the things to state precisely.
6. Say what to do when something goes wrong. A good SOP anticipates the common problem: what to do if the control fails, if the result is ambiguous, if a reagent has expired. This turns the SOP into a real working tool rather than an idealized description.

A worked illustration

To see the difference, compare a vague instruction with an SOP-quality one for the same step of a Gram stain:

Vague: "Decolorize the smear, then counterstain."

SOP-quality: "Apply decolorizer (acetone-alcohol) drop by drop until no more violet washes from the smear, about 5 to 10 seconds. Rinse immediately with water to stop decolorization. Flood with safranin counterstain and leave for 60 seconds, then rinse and blot dry."

The second version standardizes the two steps most likely to vary between operators, how long to decolorize (the hardest judgment in the whole stain) and how long to counterstain, so that two technologists produce comparable smears. That is what an SOP is for. The full procedure and principle of the Gram stain itself belongs in the technical reference; the SOP's job is to make it reproducible.

The lifecycle of an SOP: it is not written once

An SOP is a controlled, living document, not a file written once and forgotten.

It is drafted by someone who knows the procedure, reviewed by a supervisor or quality officer, and approved and signed before it becomes effective. Once effective, staff are trained against it and the training is recorded.

The SOP is then reviewed periodically (many laboratories review every one to two years) and whenever the procedure changes, a new reagent, a new instrument, an updated standard.

Each change produces a new version with a new version number and date, and the old version is withdrawn so that only the current one is in use. Superseded versions are archived, not left on the bench, because staff following an outdated SOP is a real and common error.

This control, versioning, approval, training, review, is what separates a genuine SOP from an ordinary set of instructions, and it is what an accreditation assessor examines.

How to remember

• An SOP kills variation. Its whole purpose is that the same task is done the same way by everyone, every time. If a step could be done two ways, the SOP has not finished its job.
• Control the document, not just the method. Title, number, version, date, and approval signatures are not bureaucracy; they are what make an SOP traceable and current. An unversioned instruction is not an SOP.
• Write for the competent newcomer. Enough detail that nothing critical is assumed, plain enough that it is actually followed. State every number that changes the result.
• A technical SOP without a QC section is incomplete. It must say how to know the procedure is working and what to do when the control fails.
• An SOP is alive. It is drafted, approved, trained on, reviewed, and re-versioned. The old version comes off the bench when the new one goes on.

Key facts

Where students get confused

"An SOP is just the method, like in a textbook." The method is only the core section. An SOP also has document control (number, version, approval), scope, responsibilities, safety, QC, and references, and it is approved, versioned, and reviewed. A textbook method becomes an SOP only when it is controlled and made specific to the laboratory.

"More detail is always better." No. An SOP written for an expert is too terse to standardize; one written like a textbook chapter is too long to follow and gets ignored. The target is the competent newcomer: every critical detail stated, nothing padded.

"Once written, an SOP is done." An SOP is a living controlled document. It is reviewed periodically and whenever the procedure changes, and each change produces a new approved version while the old one is withdrawn. Following a superseded SOP is a common and serious error.

"SOPs are paperwork for inspections." They are the written form of the laboratory's actual quality. Consistent results, safe practice, trained staff, and accreditation all rest on SOPs being current and followed, not on their existing to be shown to an assessor.

"A technical SOP does not need a quality-control section." It does. Without stating which controls to run, how often, and what to do when a control fails, the SOP cannot tell the user whether the procedure is working. QC is part of the procedure, not separate from it.

References

1. Clinical and Laboratory Standards Institute (CLSI). Quality Management System: Development and Management of Laboratory Documents; Approved Guideline. (QMS02.) Wayne, PA: CLSI.
2. World Health Organization. Laboratory Quality Management System: Handbook. Geneva: World Health Organization.
3. Leber AL, editor. Clinical Microbiology Procedures Handbook. 4th ed. Washington, DC: ASM Press; 2016. doi:10.1128/9781683670438.CMPH
4. International Organization for Standardization. ISO 15189: Medical laboratories — Requirements for quality and competence. Geneva: ISO.