Guidance for Industry
 

Investigating Out-of-Specification (OOS) 

Test Results for 

Pharmaceutical Production 

U.S. Department of Health and Human Services 

Food and Drug Administration 

Center for Drug Evaluation and Research (CDER) 

October 2006 

Pharmaceutical CGMPs 
 

Contains Nonbinding Recommendations 

Guidance for Industry
 

Investigating Out-of-Specification (OOS) 

Test Results for 

Pharmaceutical Production 

Additional copies are available from: 

Office of Training and Communication 

Division of Drug Information HFD-240 

Center for Drug Evaluation and Research 

Food and Drug Administration 

5600 Fishers Lane 

Rockville, MD 20857 

(Tel) 301-827-4573 

http://www.fda.gov/cder/guidance/index.htm 

U.S. Department of Health and Human Services 

Food and Drug Administration 

Center for Drug Evaluation and Research (CDER) 

October 2006 

Pharmaceutical CGMPs 
 

 

Contains Nonbinding Recommendations 

TABLE OF CONTENTS 

I.	 INTRODUCTION............................................................................................................. 1
 

II.	 BACKGROUND ............................................................................................................... 2
 

III.	 IDENTIFYING AND ASSESSING OOS TEST RESULTS — 

PHASE I: LABORATORY INVESTIGATION ........................................................... 3
 

A. Responsibility of the Analyst.........................................................................................................4
 

B. Responsibilities of the Laboratory Supervisor............................................................................4
 

IV.	 INVESTIGATING OOS TEST RESULTS — 

PHASE II: FULL-SCALE OOS INVESTIGATION ................................................... 6
 

A. Review of Production.....................................................................................................................6
 

B. Additional Laboratory Testing.....................................................................................................7
 

C. Reporting Testing Results.............................................................................................................9
 

V. CONCLUDING THE INVESTIGATION.................................................................... 12
 

A. Interpretation of Investigation Results ......................................................................................12
 

B. Cautions........................................................................................................................................14
 

C. Field Alert Reports ......................................................................................................................14
 
 

 

Contains Nonbinding Recommendations 

GUIDANCE FOR INDUSTRY1 

Investigating Out-of-Specification (OOS) Test Results 

for Pharmaceutical Production 

This guidance represents the Food and Drug Administration's (FDA's) current thinking on this topic. It 

does not create or confer any rights for or on any person and does not operate to bind FDA or the public. 

You can use an alternative approach if the approach satisfies the requirements of the applicable statutes 

and regulations. If you want to discuss an alternative approach, contact the FDA staff responsible for 

implementing this guidance. If you cannot identify the appropriate FDA staff, call the appropriate 

number listed on the title page of this guidance. 

I. INTRODUCTION 

This guidance for industry provides the Agency’s current thinking on how to evaluate out-ofspecification (OOS) test results. For purposes of this document, the term OOS results includes 

all test results that fall outside the specifications or acceptance criteria established in drug 

applications, drug master files (DMFs), official compendia, or by the manufacturer. The term 

also applies to all in-process laboratory tests that are outside of established specifications.2 

This guidance applies to chemistry-based laboratory testing of drugs regulated by CDER. It is 

directed toward traditional drug testing and release methods. These laboratory tests are 

performed on active pharmaceutical ingredients, excipients and other components, in-process 

materials, and finished drug products3

 to the extent that current good manufacturing practice 

(CGMP) regulations (21 CFR parts 210 and 211) and the Federal Food, Drug, and Cosmetic Act 

(the Act) (section 501(a)(2)(B)) apply. The principles in this guidance also apply to in-house 

testing of drug product components that are purchased by a firm. This guidance can also be used 

by contract firms performing production and/or laboratory testing responsibilities. Specifically, 

the guidance discusses how to investigate OOS test results, including the responsibilities of 

1

 This guidance has been prepared by the Office of Compliance/Division of Manufacturing and Product Quality in 

the Center for Drug Evaluation and Research (CDER). 

2

 In certain instances, in-process testing is done solely for purposes of triggering real time equipment or system 

adjustments to prevent process drift. This guidance does not address these situations. 

3

 Chemistry-based laboratory testing of biotechnology products that are under the jurisdiction of CDER are within 

the scope of this guidance. However, this guidance is not intended to address biological assays (e.g., in vivo, 

immunoassays). 

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laboratory personnel, the laboratory phase of the investigation, additional testing that may be 

necessary, when to expand the investigation outside the laboratory, and the final evaluation of all 

test results. 

The Agency, in accordance with its August 2002 “Pharmaceutical CGMPs for the 21st Century” 

initiative, encourages modern approaches to manufacturing, monitoring, and control to enhance 

process predictability and efficiency. Process Analytical Technology (PAT) takes a different 

approach to quality assurance by using process controls and in-process data as the release 

specification instead of relying on single laboratory determinations to make batch acceptability 

decisions. This guidance is not intended to address PAT approaches, as routine in-process use of 

these methods might include other considerations. For information on timely in-process testing, 

see the CGMP guidance entitled PAT — A Framework for Innovative Pharmaceutical 

Development, Manufacturing, and Quality Assurance. 

FDA's guidance documents, including this guidance, do not establish legally enforceable 

responsibilities. Instead, guidances describe the Agency's current thinking on a topic and should 

be viewed only as recommendations, unless specific regulatory or statutory requirements are 

cited. The use of the word should in Agency guidances means that something is suggested or 

recommended, but not required. 

II. BACKGROUND 

Laboratory testing, which is required by the CGMP regulations (§§ 211.160 and 211.165), is 

necessary to confirm that components, containers and closures, in-process materials, and finished 

products conform to specifications, including stability specifications. 

Testing also supports analytical and process validation efforts.4 General CGMP regulations 

covering laboratory operations can be found in part 211, subparts I (Laboratory Controls) and J 

(Records and Reports). These regulations provide for the establishment of scientifically sound 

and appropriate specifications, standards, and test procedures that are designed to ensure that 

components, containers and closures, in-process materials, and finished drug products conform 

to the established standards. Section 211.165(f) of the CGMP regulations specifies that finished 

drug products that fail to meet established standards, specifications, or other relevant quality 

control criteria will be rejected. 

Both finished pharmaceuticals and active pharmaceutical ingredients (APIs) are to be 

manufactured in accordance with current good manufacturing practice under section 

4 Specifications must be scientifically sound and appropriate (§ 211.160(b)), test procedures must be validated as to 

their accuracy, sensitivity, specificity, and reproducibility (§ 211.165(e)), and the suitability of the test procedures 

under actual conditions of use must be documented (§ 211.194(a)(2)). For products that are the subjects of new drug 

applications (NDAs), abbreviated new drug applications (ANDAs), or investigational new drug applications (INDs), 

specifications are contained in the application or DMF. Specifications for nonapplication products may be found in 

official compendia or established by the manufacturer. 

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Contains Nonbinding Recommendations 

501(a)(2)(B) of the Act. Current good manufacturing practice for APIs includes the performance 

of scientifically sound raw material testing, in-process monitoring, release and stability testing, 

process validation, and adequate investigations of any OOS result obtained from such testing. 

All citations to part 211 in this document pertain to finished pharmaceuticals, but these 

referenced regulatory requirements are also consistent with Agency guidance on CGMPs for 

APIs with respect to laboratory controls, which include out-of-specification investigations. See 

FDA’s guidance for industry Q7A Good Manufacturing Practice Guidance for Active 

Pharmaceutical Ingredients (ICH Q7A) for specific recommendations.5 

The responsibility of a contract testing laboratory in meeting these requirements is equivalent to 

that of a manufacturing firm. 

III. IDENTIFYING AND ASSESSING OOS TEST RESULTS — 

PHASE I: LABORATORY INVESTIGATION 

FDA regulations require that an investigation be conducted whenever an OOS test result is 

obtained (§ 211.192).6 The purpose of the investigation is to determine the cause of the OOS 

result. The source of the OOS result should be identified either as an aberration of the 

measurement process or an aberration of the manufacturing process. Even if a batch is rejected 

based on an OOS result, the investigation is necessary to determine if the result is associated 

with other batches of the same drug product or other products. Batch rejection does not negate 

the need to perform the investigation. The regulations require that a written record of the 

investigation be made, including the conclusions and follow-up (§ 211.192). 

To be meaningful, the investigation should be thorough, timely, unbiased, well-documented, and 

scientifically sound. The first phase of such an investigation should include an initial assessment 

of the accuracy of the laboratory's data. Whenever possible, this should be done before test 

preparations (including the composite or the homogenous source of the aliquot tested) are 

discarded. This way, hypotheses regarding laboratory error or instrument malfunctions can be 

tested using the same test preparations. If this initial assessment indicates that no meaningful 

errors were made in the analytical method used to arrive at the data, a full-scale OOS 

investigation should be conducted. For contract laboratories, the laboratory should convey its 

data, findings, and supporting documentation to the manufacturing firm’s quality control unit 

(QCU), who should then initiate the full-scale OOS investigation. 

5

 We update guidances periodically. To make sure you have the most recent version of a guidance, check the CDER 

guidance page at http://www.fda.gov/cder/guidance/index.htm. 

6 Although the subject of this document is OOS results, much of the guidance may be useful for examining results 

that are out of trend. 

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A. Responsibility of the Analyst 

The first responsibility for achieving accurate laboratory testing results lies with the analyst who 

is performing the test. The analyst should be aware of potential problems that could occur 

during the testing process and should watch for problems that could create inaccurate results. 

In accordance with the CGMP regulations in § 211.160 (b)(4), the analyst should ensure that 

only those instruments meeting established performance specifications are used and that all 

instruments are properly calibrated. 

Certain analytical methods have system suitability requirements, and systems not meeting these 

requirements should not be used. For example, in chromatographic systems, reference standard 

solutions may be injected at intervals throughout chromatographic runs to measure drift, noise, 

and repeatability. If reference standard responses indicate that the system is not functioning 

properly, all of the data collected during the suspect time period should be properly identified 

and should not be used. The cause of the malfunction should be identified and, if possible, 

corrected before a decision is made whether to use any data prior to the suspect period. 

Analysts should check the data for compliance with test specifications before discarding test 

preparations or standard preparations. When unexpected results are obtained and no obvious 

explanation exists, test preparations should be retained, if stable, and the analyst should inform 

the supervisor. An assessment of the accuracy of the results should be started immediately. 

If errors are obvious, such as the spilling of a sample solution or the incomplete transfer of a 

sample composite, the analyst should immediately document what happened. Analysts should 

not knowingly continue an analysis they expect to invalidate at a later time for an assignable 

cause (i.e., analyses should not be completed for the sole purpose of seeing what results can be 

obtained when obvious errors are known). 

B. Responsibilities of the Laboratory Supervisor 

Once an OOS result has been identified, the supervisor's assessment should be objective and 

timely. There should be no preconceived assumptions as to the cause of the OOS result. Data 

should be assessed promptly to ascertain if the results might be attributed to laboratory error, or 

whether the results could indicate problems in the manufacturing process. An immediate 

assessment could include re-examination of the actual solutions, test units, and glassware used in 

the original measurements and preparations, which might provide more credibility for laboratory 

error hypotheses. 

The following steps should be taken as part of the supervisor's assessment: 

1. Discuss the test method with the analyst; confirm analyst knowledge of and 

performance of the correct procedure. 

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2. Examine the raw data obtained in the analysis, including chromatograms and 

spectra, and identify anomalous or suspect information. 

3. Verify that the calculations used to convert raw data values into a final test result 

are scientifically sound, appropriate, and correct; also determine if unauthorized 

or unvalidated changes have been made to automated calculation methods. 

4. Confirm the performance of the instruments. 

5. Determine that appropriate reference standards, solvents, reagents, and other 

solutions were used and that they met quality control specifications. 

6. Evaluate the performance of the test method to ensure that it is performing 

according to the standard expected based on method validation data and historical 

data. 

7. Fully document and preserve records of this laboratory assessment. 

The assignment of a cause for OOS results will be greatly facilitated if the retained sample 

preparations are examined promptly. Hypotheses regarding what might have happened (e.g. 

dilution error, instrument malfunction) should be tested. Examination of the retained solutions 

should be performed as part of the laboratory investigation. 

Examples: 

•	 Solutions can be re-injected as part of an investigation where a transient equipment 

malfunction is suspected. Such hypotheses are difficult to prove. However, reinjections 

can provide strong evidence that the problem should be attributed to the instrument, 

rather than the sample or its preparation. 

•	 For release rate testing of certain specialized dosage form drugs that are not destroyed 

during testing, where possible, examination of the original dosage unit tested might 

determine whether it was damaged during laboratory handling in a way that affected its 

performance. Such damage would provide evidence to invalidate the OOS test result, 

and a retest would be indicated. 

•	 Further extraction of a dosage unit, where possible, can be performed to determine 

whether it was fully extracted during the original analysis. Incomplete extraction could 

invalidate the test results and should lead to questions regarding validation of the test 

method. 

It is important that each step in the investigation be fully documented. Laboratory management 

should ascertain not only the reliability of the individual value obtained, but also the significance 

these OOS results represent to the laboratory quality assurance program. Laboratory 

management should be especially alert to developing trends. As part of an effective quality 

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system, a firm’s upper management should appropriately monitor these trends and ensure that 

any problematic areas are addressed. 

Laboratory error should be relatively rare. Frequent errors suggest a problem that might be due 

to inadequate training of analysts, poorly maintained or improperly calibrated equipment, or 

careless work. Whenever laboratory error is identified, the firm should determine the source of 

that error and take corrective action to prevent recurrence. To ensure full compliance with the 

CGMP regulations, the manufacturer also should maintain adequate documentation of the 

corrective action. 

In summary, when clear evidence of laboratory error exists, laboratory testing results should be 

invalidated. When evidence of laboratory error remains unclear, a full-scale OOS investigation 

should be conducted by the manufacturing firm to determine what caused the unexpected results. 

It should not be assumed that OOS test results are attributable to analytical error without 

performing and documenting an investigation. Both the initial laboratory assessment and the 

following OOS investigation should be documented fully. 

IV. INVESTIGATING OOS TEST RESULTS — 

PHASE II: FULL-SCALE OOS INVESTIGATION 

When the initial assessment does not determine that laboratory error caused the OOS result and 

testing results appear to be accurate, a full-scale OOS investigation using a predefined procedure 

should be conducted. This investigation may consist of a production process review and/or 

additional laboratory work. The objective of such an investigation should be to identify the root 

cause of the OOS result and take appropriate corrective and preventative action.7 A full-scale 

investigation should include a review of production and sampling procedures, and will often 

include additional laboratory testing. Such investigations should be given the highest priority. 

Among the elements of this phase is evaluation of the impact of OOS result(s) on already 

distributed batches. 

A.	 Review of Production 

The investigation should be conducted by the QCU and should involve all other departments that 

could be implicated, including manufacturing, process development, maintenance, and 

engineering. In cases where manufacturing occurs off-site (i.e., performed by a contract 

manufacturer or at multiple manufacturing sites), all sites potentially involved should be 

included in the investigation. Other potential problems should be identified and investigated. 

7

 Please note that § 211.192 requires a thorough investigation of any discrepancy, including documentation of 

conclusions and follow-up. Implicit in this requirement for investigation is the need to implement corrective and 

preventative actions. Corrective and preventive action is consistent with the FDA’s requirements under 21 CFR part 

820, subpart J, pertaining to medical devices, as well as the 2004 draft guidance entitled Quality Systems Approach 

to Pharmaceutical Current Good Manufacturing Practice Regulations, which, when finalized, will represent the 

Agency’s current thinking on this topic. 

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The records and documentation of the manufacturing process should be fully reviewed to 

determine the possible cause of the OOS result(s). 

A full-scale OOS investigation should consist of a timely, thorough, and well-documented 

review. A written record of the review should include the following information. 

1. A clear statement of the reason for the investigation. 

2. A summary of the aspects of the manufacturing process that may have caused the 

problem. 

3. The results of a documentation review, with the assignment of actual or probable 

cause. 

4. The results of a review made to determine if the problem has occurred previously. 

5. A description of corrective actions taken. 

If this part of the OOS investigation confirms the OOS result and is successful in identifying its 

root cause, the OOS investigation may be terminated and the product rejected. However, a 

failure investigation that extends to other batches or products that may have been associated with 

the specific failure must be completed (§ 211.192). If any material was reprocessed after 

additional testing, the investigation should include comments and the signatures of appropriate 

production and quality control personnel. 

OOS results may indicate a flaw in product or process design. For example, a lack of robustness 

in product formulation, inadequate raw material characterization or control, substantial variation 

introduced by one or more unit operations of the manufacturing process, or a combination of 

these factors can be the cause of inconsistent product quality. In such cases, it is essential that 

redesign of the product or process be undertaken to ensure reproducible product quality.8 

B. Additional Laboratory Testing 

A full-scale OOS investigation may include additional laboratory testing. A number of practices 

are used during the laboratory phase of an investigation. These include (1) retesting a portion of 

the original sample and (2) resampling. 

1. Retesting 

Part of the investigation may involve retesting of a portion of the original sample. The 

sample used for the retesting should be taken from the same homogeneous material that 

was originally collected from the lot, tested, and yielded the OOS results. For a liquid, it 

8

 OOS results might also be the result of the objectionable practice of making unauthorized or unvalidated changes to 

the manufacturing process. 

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may be from the original unit liquid product or composite of the liquid product; for a 

solid, it may be an additional weighing from the same sample composite prepared for the 

original test. 

Situations where retesting is indicated include investigating testing instrument 

malfunctions or to identify a possible sample handling problem, for example, a suspected 

dilution error. Decisions to retest should be based on the objectives of the testing and 

sound scientific judgment. It is often important for the predefined retesting plan to 

include retests performed by an analyst other than the one who performed the original 

test. A second analyst performing a retest should be at least as experienced and qualified 

in the method as the original analyst. 

The CGMP regulations require the establishment of specifications, standards, sampling 

plans, test procedures, and other laboratory control mechanisms (§ 211.160). 

FDA inspections have revealed that some firms use a strategy of repeated testing until a 

passing result is obtained, then disregarding the OOS results without scientific 

justification. This practice of “testing into compliance” is unscientific and objectionable 

under CGMPs. The maximum number of retests to be performed on a sample should be 

specified in advance in a written standard operating procedure (SOP). The number may 

vary depending upon the variability of the particular test method employed, but should be 

based on scientifically sound principles. The number of retests should not be adjusted 

depending on the results obtained. The firm's predetermined retesting procedures should 

contain a point at which the additional testing ends and the batch is evaluated. If the 

results are unsatisfactory at this point, the batch is suspect and must be rejected or held 

pending further investigation (§ 211.165(f)). Any deviation from this SOP should be rare 

and done in accordance with § 211.160(a), which states that any deviations from written 

specifications, sampling plans, test procedures, or other laboratory control mechanisms 

shall be recorded and justified. In such cases, before starting additional retesting, a 

protocol should be prepared (subject to approval by the QCU) that describes the 

additional testing to be performed and specifies the scientific and/or technical handling of 

the data. 

In the case of a clearly identified laboratory error, the retest results would substitute for 

the original test result. All original data should be retained, however, and an explanation 

recorded. This record should be initialed and dated by the involved persons and include a 

discussion of the error and supervisory comments. (See section III of this guidance for 

more details on a laboratory investigation.) 

If no laboratory or calculation errors are identified in the first test, there is no scientific 

basis for invalidating initial OOS results in favor of passing retest results. All test results, 

both passing and suspect, should be reported 9

 and considered in batch release decisions. 

9

 In other words, all data are reported in, for example, quality control reports, batch records, Certificates of Analysis, 

in accordance with §§ 211.188 and 211.192. 

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2. Resampling 

While retesting refers to analysis of the original, homogenous sample material, 

resampling involves analyzing a specimen from any additional units collected as part of 

the original sampling procedure or from a new sample collected from the batch, should 

that be necessary. 

The original sample from a batch should be sufficiently large to accommodate additional 

testing in the event an OOS result is obtained. In some situations, however, it may be 

appropriate to collect a new sample from the batch. Control mechanisms for examination 

of additional specimens should be in accordance with predetermined procedures and 

sampling strategies (§ 211.165(c)). 

When all data have been evaluated, an investigation might conclude that the original 

sample was prepared improperly and was therefore not representative of the batch quality 

(§ 211.160(b)(3)). Improper sample preparation might be indicated, for example, by 

widely varied results obtained from several aliquots of an original composite (after 

determining there was no error in the performance of the analysis). Resampling should 

be performed by the same qualified, validated methods that were used for the initial 

sample. However, if the investigation determines that the initial sampling method was 

inherently inadequate, a new accurate sampling method must be developed, documented, 

and reviewed and approved by the QCU (§§ 211.160 and 211.165(c)). 

C. Reporting Testing Results 

Practices used in reporting and interpretation of test results include (1) averaging and (2) outlier 

tests. 

1. Averaging 

There are both appropriate and inappropriate uses of averaging test data during original 

testing and during an OOS investigation: 

a. Appropriate uses 

Averaging data can be a valid approach, but its use depends upon the sample and its 

purpose. For example, in an optical rotation test, several discrete measurements are 

averaged to determine the optical rotation for a sample, and this average is reported as the 

test result. If the sample can be assumed to be homogeneous, (i.e., an individual sample 

preparation designed to be homogenous), using averages can provide a more accurate 

result. In the case of microbiological assays, the U.S. Pharmacopeia (USP) prefers the use 

of averages because of the innate variability of the biological test system. 

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It should be noted that a test might consist of a specific number of replicates to arrive at a 

result. For instance, an HPLC assay result may be determined by averaging the peak 

responses from a number of consecutive, replicate injections from the same preparation 

(usually 2 or 3). The assay result would be calculated using the peak response average. 

This determination is considered one test and one result. This is a distinct difference from 

the analysis of different portions from a lot, intended to determine variability within the 

lot, and from multiple full analyses of the same homogenous sample. The use of 

replicates to arrive at a single reportable10 result, and the specific number of replicates 

used, should be specified in the written, approved test method. Acceptance limits for 

variability among the replicates should also be specified in the method. Unexpected 

variation in replicate determinations should trigger remedial action as required by 

§ 211.160(b)(4). If acceptance limits for replicate variability are not met, the test results 

should not be used. 

In some cases, a series of complete tests (full run-throughs of the test procedure), such as 

assays, are part of the test method. It may be appropriate to specify in the test method that 

the average of these multiple assays is considered one test and represents one reportable 

result. In this case, limits on acceptable variability among the individual assay results 

should be based on the known variability of the method and should also be specified in 

the test methodology. A set of assay results not meeting these limits should not be used. 

These appropriate uses of averaging test data should be used during an OOS investigation 

only if they were used during the original testing that produced the OOS result. 

b. Inappropriate uses 

Reliance on averaging has the disadvantage of hiding variability among individual test 

results. For this reason, all individual test results should normally be reported as separate 

values. Where averaging of separate tests is appropriately specified by the test method, a 

single averaged result can be reported as the final test result. In some cases, a statistical 

treatment of the variability of results is reported. For example, in a test for dosage form 

content uniformity, the standard deviation (or relative standard deviation) is reported 

with the individual unit dose test results. 

Averaging can also conceal variations in different portions of a batch, or within a sample. 

For example, the use of averages is inappropriate when performing powder blend/mixture 

uniformity or dosage form content uniformity determinations. In these cases, testing is 

intended to measure variability within the product, and individual results provide the 

information for such an evaluation. 

In the context of additional testing performed during an OOS investigation, averaging the 

result(s) of the original test that prompted the investigation and additional retest or 

10 The term reportable result as used in this document means a final analytical result. This result is appropriately 

defined in the written approved test method and derived from one full execution of that method, starting from the 

original sample. 

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resample results obtained during the OOS investigation is not appropriate because it 

hides variability among the individual results. Relying on averages of such data can be 

particularly misleading when some of the results are OOS and others are within 

specifications. It is critical that the laboratory provide all individual results for 

evaluation and consideration by the QCU, which is responsible for approving or 

rejecting, e.g., drug products, in-process materials (§ 211.22) 

For example, in an assay of a finished drug with a specification of 90 to 110 percent, an 

initial OOS result of 89 percent followed by additional retest results of 90 percent and 91 

percent would produce an average of 90 percent. While this average would meet 

specifications,11 the additional test results also tend to confirm the original OOS result. 

However, in another situation with the same specifications, an initial OOS result of 80 

percent followed by additional test results of 85 percent and 105 percent would also 

produce an average of 90 percent, but present a much different picture. These results do 

not confirm the original OOS result but show high variability and may not be reliable. In 

both examples, the individual results, not the average, should be used to evaluate the 

quality of the product. 

2. Outlier Tests 

The CGMP regulations require that statistically valid quality control criteria include 

appropriate acceptance and/or rejection levels (§ 211.165(d)). On rare occasions, a value 

may be obtained that is markedly different from the others in a series obtained using a 

validated method. Such a value may qualify as a statistical outlier. An outlier may result 

from a deviation from prescribed test methods, or it may be the result of variability in the 

sample. It should never be assumed that the reason for an outlier is error in the testing 

procedure, rather than inherent variability in the sample being tested. 

Outlier testing is a statistical procedure for identifying from an array those data that are 

extreme. The possible use of outlier tests should be determined in advance. This should 

be written into SOPs for data interpretation and be well documented. The SOPs should 

include the specific outlier test to be applied with relevant parameters specified in 

advance. The SOPs should specify the minimum number of results required to obtain a 

statistically significant assessment from the specified outlier test. 

For biological assays having a high variability, an outlier test may be an appropriate 

statistical analysis to identify those results that are statistically extreme observations. 

The USP describes outlier tests in the general chapter on Design and Analysis of 

Biological Assays <111>.

12 In these cases, the outlier observation is omitted from 

11 When arriving at a batch disposition decision, it is important for a firm to assess whether the low assay value may 

project to a subpotency failure before the product’s labeled expiration date. 

12 The United States Pharmacopeia, 29th Revision, Rockville, MD: The United States Pharmacopeial Convention, 

2006. 

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calculations. The USP also states that “arbitrary rejection or retention of an apparently 

aberrant response can be a serious source of bias... the rejection of observations solely on 

the basis of their relative magnitudes is a procedure to be used sparingly” (USP <111>). 

For validated chemical tests with relatively small variance, and if the sample being tested 

can be considered homogeneous (for example, an assay of a composite of a dosage form 

drug to determine strength), an outlier test is only a statistical analysis of the data 

obtained from testing and retesting. It will not identify the cause of an extreme 

observation and, therefore, should not be used to invalidate the suspect result. 

Occasionally, an outlier test may be of some value in estimating the probability that the 

OOS result is discordant from a data set, and this information can be used in an auxiliary 

fashion, along with all other data from the investigation, to evaluate the significance of 

the result. 

Outlier tests have no applicability in cases where the variability in the product is what is 

being assessed, such as for content uniformity, dissolution, or release rate determinations. 

In these applications, a value perceived to be an outlier may in fact be an accurate result 

of a nonuniform product. 

When using these practices during the additional testing performed in an OOS investigation, the 

laboratory will obtain multiple results. It is again critical for the laboratory to provide all test 

results for evaluation and consideration by the QCU in its final disposition decision. In addition, 

when investigation by a contract laboratory13 does not determine an assignable cause, all test 

results should be reported to the customer on the certificate of analysis. 

V. CONCLUDING THE INVESTIGATION 

To conclude the investigation, the results should be evaluated, the batch quality should be 

determined, and a release decision should be made by the QCU. The SOPs should be followed 

in arriving at this point. Once a batch has been rejected, there is no limit to further testing to 

determine the cause of the failure so that a corrective action can be taken. 

A. Interpretation of Investigation Results 

The QCU is responsible for interpreting the results of the investigation. An initial OOS result 

does not necessarily mean the subject batch fails and must be rejected. The OOS result should be 

investigated, and the findings of the investigation, including retest results, should be interpreted 

to evaluate the batch and reach a decision regarding release or rejection (§ 211.165). 

In those instances where an investigation has revealed a cause, and the suspect result is 

invalidated, the result should not be used to evaluate the quality of the batch or lot. Invalidation 

13 The Agency also recommends that OOS investigation reports be provided to the customer. 

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of a discrete test result may be done only upon the observation and documentation of a test event 

that can reasonably be determined to have caused the OOS result. 

In those cases where the investigation indicates an OOS result is caused by a factor affecting the 

batch quality (i.e., an OOS result is confirmed), the result should be used in evaluating the 

quality of the batch or lot. A confirmed OOS result indicates that the batch does not meet 

established standards or specifications and should result in the batch's rejection, in accordance 

with § 211.165(f), and proper disposition. For inconclusive investigations — in cases where an 

investigation (1) does not reveal a cause for the OOS test result and (2) does not confirm the 

OOS result — the OOS result should be given full consideration in the batch or lot disposition 

decision. 

In the first case (OOS confirmed), the investigation changes from an OOS investigation into a 

batch failure investigation, which must be extended to other batches or products that may have 

been associated with the specific failure (§ 211.192). 

In the second case (inconclusive), the QCU might still ultimately decide to release the batch. For 

example, a firm might consider release of the product under the following scenario: 

A product has an acceptable composite assay range of 90.0 to 110.0 percent. The initial (OOS) 

assay result is 89.5 percent. Subsequent sample preparations from the original sample yield the 

following retest results: 99.0, 98.9, 99.0, 99.1, 98.8, 99.1, and 99.0 percent. A comprehensive 

laboratory investigation (Phase 1) fails to reveal any laboratory error. Review of events during 

production of the batch reveals no aberrations or indication of unusual process variation.14 

Review of the manufacturing process and product history demonstrates that the process is robust. 

The seven passing retest results are all well within the known limits of variability of the method 

used. Batch results from in-process monitoring, content uniformity, dissolution, and other tests 

are consistent with the passing retest results. After a thorough investigation, a firm’s QCU 

might conclude that the initial OOS result did not reflect the true quality of the batch. 

It is noteworthy in this scenario that the original, thorough laboratory investigation failed to find 

any assignable cause. However, if subsequent investigation nonetheless concludes that the 

source of the OOS result was a cause unrelated to the manufacturing process, in response to this 

atypical failure to detect the laboratory deviation, it is essential that the investigation include 

appropriate follow-up and scrutiny to prevent recurrence of the laboratory error(s) that could 

have led to the OOS result. 

As the above example illustrates, any decision to release a batch, in spite of an initial OOS result 

that has not been invalidated, should come only after a full investigation has shown that the OOS 

result does not reflect the quality of the batch. In making such a decision, the QCU should 

always err on the side of caution. 

14 As an example, evaluation of process variation would determine if established equipment, facility, and process 

control limits were met. 

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Contains Nonbinding Recommendations 

B. Cautions 

In cases where a series of assay results (to produce a single reportable result) are required by the 

test procedure and some of the individual results are OOS, some are within specification, and all 

are within the known variability of the method, the passing results are no more likely to 

represent the true value for the sample than the OOS results. For this reason, a firm should err on 

the side of caution and treat the reportable average of these values as an OOS result, even if that 

average is within specification. This approach is consistent with the principle outlined in the 

USP General Notices that an official article shall comply with the compendial standard any time 

a compendial test is applied.15 Thus, every individual application of the official test should be 

expected to produce a result that meets specifications. 

An assay result that is low, but within specifications, should also raise a concern. One cause of 

the result could be that the batch was not formulated properly. Batches must be formulated with 

the intent to provide not less than 100 percent of the labeled or established amount of active 

ingredient (§ 211.101 (a)). This would also be a situation where the analytical result meets 

specifications, but caution should be used in the release or reject decision. 

As with all analytical testing conducted to evaluate the quality of a drug, all records pertaining to 

the OOS test result should be retained. Records must be kept of complete data derived from all 

tests performed to ensure compliance with established specifications and standards (§ 211.194). 

C. Field Alert Reports 

For those products that are the subject of approved full and abbreviated new drug applications, 

regulations require submitting within 3 working days a field alert report (FAR) of information 

concerning any failure of a distributed batch to meet any of the specifications established in an 

application (21 CFR 314.81(b)(1)(ii)). OOS test results on these products are considered to be 

one kind of "information concerning any failure” described in this regulation. Unless the OOS 

result on the distributed batch is found to be invalid within 3 days, an initial FAR should be 

submitted. A follow-up FAR should be submitted when the OOS investigation is completed. 

15 See USP 29, General Notices, “Test Results, Statistics, and Standards.” 

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