SOP: Data Integrity Controls for Manual Microbiological Plate Reading and Counting

This is a ready-to-use SOP for the single most common act in a micro lab: counting a plate by eye and recording the result. Replace every <<FILL: ...>> placeholder with your own specifics, set your document numbers and dates, and route it through your normal document control, review, and approval. A worked filled specimen follows the template so you can see how a completed version reads. This describes a general control approach in original wording; it is not a reproduction of any compendial chapter, and it is general guidance to adapt, not legal or regulatory advice. Verify each cited regulation against the current source before you rely on it.

Document control header

1. Purpose

This procedure defines how <<FILL: COMPANY NAME>> records, counts, and verifies manually read microbiological plates, filters, and tubes so that the resulting count is contemporaneous, attributable, reproducible, and independently confirmed while the physical evidence still exists. The objective is to close the gap that makes manual microbiology counts harder to defend than instrument-generated data: once a plate is discarded, the count and the check on the count are the only evidence that will ever exist.

2. Scope

This procedure applies to manual colony counting, turbidity reading, and morphological scoring performed on environmental monitoring plates, bioburden plates and filters, sterility test observations, water system plates, and any other compendial or in-house microbiological plate or tube read by eye at the sites listed in the header. It covers the point of observation through disposal of the plate, and the retention of any image taken of it. It does not replace the assay-specific test procedures (for example the site’s sterility testing or environmental monitoring program SOPs), which define the method itself; this procedure defines the data integrity controls that apply across all of them. Where an imaging or automated colony counter is used, this procedure applies to the review and verification of its output; the instrument’s own operating instructions govern its use.

3. Responsibilities

4. Definitions

• Contemporaneous recording: recording an observation at the moment it is made, by the person who made it, with the real clock time, rather than from memory at a later point.
• Second-person verification: an independent, qualified analyst examining the same physical plate, or a retained image of it, and confirming the reported count before the plate is discarded.
• Too numerous to count (TNTC): a plate whose growth exceeds the countable range defined by the method, reported per the estimation or dilution-repeat rule in section 5.4, never as a guessed exact number.
• Countable range: the range of colony counts, defined in the method, within which a manual count is considered reliable.

5. Procedure

5.1 Record at the point of observation

1. Bring the controlled recording form to the reading station before beginning any counts; do not read plates first and record afterward.
2. Count each plate or filter individually and record its result, against its specific sample and plate identifier, before moving to the next plate.
3. Record the real clock time of each individual read. A single batch time applied to a set of plates read over several minutes is not acceptable.
4. Where the reading station is physically separate from where the form is normally kept, the form travels to the reading station; do not create a transcribe-later step.

5.2 Apply a documented counting method

1. Use the site’s documented counting method for the plate type being read, including how a colony is defined, how merged or confluent growth is handled, and how spreaders, satellites, or artifacts are distinguished from true colonies.
2. Record the count using the reporting convention and units defined in the method (CFU per plate, per filter, per cubic meter, or per contact area, as applicable).
3. Where an imaging or automated colony counter is used, apply the same documented method to any manual override, and record the reason for the override.

5.3 Perform second-person verification before discard

1. Route the plate, uncounted, to a second qualified analyst, or have the second analyst examine the plate immediately after the first count.
2. The second analyst independently examines the physical plate itself, not only the written number, and confirms the count or records a discrepancy.
3. This verification must occur before the plate is discarded. Verification performed after disposal, against the written number alone, does not satisfy this step and must not be recorded as complete.
4. Where the two counts differ beyond the tolerance defined in <<FILL: tolerance, e.g. +/- 5 percent for counts above 100 CFU>>, resolve per section 5.5.
5. The second analyst signs and dates the verification, adjacent to the original count.

5.4 Handle estimated and too-numerous-to-count plates

1. Where growth exceeds the countable range, do not report an estimated exact number as if it were a direct count.
2. Report the result per the method’s defined TNTC or estimation convention (for example, reporting as “TNTC” with a qualitative description, or repeating the test at a higher dilution).
3. Photograph any TNTC or near-limit plate before disposal, per section 5.6.
4. Record the basis for the reported value (TNTC designation, or the dilution used for the repeat) on the controlled record.

5.5 Resolve counting discrepancies

1. Where the first count and the verification count differ beyond the defined tolerance, re-examine the plate together, or capture an image and escalate to a third qualified reviewer.
2. Document the original count, the verification count, the resolution method, and the final reported value with its basis.
3. Do not silently overwrite either original count. Both counts and the resolution remain part of the record.

5.6 Image and retain where required

1. Photograph any plate that is an excursion, a near-limit result, a TNTC result, a sterility-positive observation, or a result feeding a recovered isolate, per <<FILL: imaging trigger list, or reference to the site's imaging procedure>>.
2. Name and store the image so it ties unambiguously to the plate, sample, date, and analyst.
3. Hold the physical plate for the defined retention window, <<FILL: retention window, e.g. 48 to 72 hours>>, before disposal, in case a re-read or challenge arises.

6. Acceptance criteria

A manual count is acceptable when it was recorded at the moment of observation by the counter, using a documented counting method, with an independent second-person verification performed against the physical plate or a retained image before the plate was discarded, any discrepancy resolved and documented against physical evidence, any TNTC or estimated result reported per the defined convention rather than as a guessed number, and any required image captured and traceable to the plate, sample, date, and analyst.

7. Records generated

• Controlled bench recording form or electronic equivalent, with contemporaneous count, time, and initials.
• Second-person verification record, with time, initials, and any discrepancy noted.
• Retained plate images, where imaging is triggered.
• Discrepancy resolution records, where applicable.

8. References

21 CFR 211.194 (laboratory records) and 211.160(b) (scientifically sound laboratory controls). USP <61> Microbial Enumeration Tests and USP <62> Tests for Specified Microorganisms (general approach to enumeration methods; consult the current chapters directly for method detail). USP <1117> Microbiological Best Laboratory Practices. FDA guidance, Data Integrity and Compliance With Drug CGMP, Questions and Answers (December 2018). MHRA GXP Data Integrity Guidance and Definitions (March 2018). PIC/S PI 041, Good Practices for Data Management and Integrity in Regulated GMP/GDP Environments.

Confirm the current version and clause numbers of each reference before issue.

9. Revision history

10. Approvals

Filled specimen

The following shows a completed bench record for an example bioburden sample, so you can see the level of detail an inspector expects. The company, sample, and numbers are illustrative; replace them with your own.

The verification here happened while the plate was still in front of both analysts, the tolerance rule was applied rather than the two analysts simply agreeing to pick a number, and the basis for the reported value is stated rather than implied.

Common inspection findings this SOP prevents

• Plate counts written on scratch paper and transcribed to the controlled form later, so the form is a copy of a copy.
• Second-person “verification” performed after the plate was already discarded, against the written number only.
• No documented counting method, so two analysts count the same plate differently and neither result can be shown to be right.
• A too-numerous-to-count plate reported as a specific estimated number with no stated basis.
• Discrepant counts silently overwritten to agree, with no record of the original disagreement.

How to adapt this SOP

1. Set your document number, owner, and effective date in the header.
2. Insert your site’s actual countable ranges, tolerances, and TNTC/estimation conventions in sections 5.2 and 5.4; these vary by method and matrix.
3. Point the imaging trigger list in section 5.6 to your site’s actual imaging procedure or inventory of imaged result types.
4. If an imaging or automated colony counter is in use, add a cross-reference to its own operating and verification procedure, and confirm section 5.2’s override-and-reason requirement is reflected there.
5. Confirm every regulation and compendial chapter in section 8 against the current published version before issue.