Compliance Policy for the 
Quantity of Bioavailability and 
Bioequivalence Samples 
Retained Under 21 CFR 
320.38(c)  
Guidance for Industry 
 
 
 
This guidance is for immediate implementation.
 
 
 
FDA is issuing this guidance for immediate implementation in accordance with 21 CFR 
10.115(g)(2).  Submit one set of either electronic or written comments on this guidance at any 
time.  Submit electronic comments to https://www.regulations.gov.  Submit written comments to 
the Dockets Management Staff (HFA-305), Food and Drug Administration, 5630 Fishers Lane, 
Rm. 1061, Rockville, MD 20852.  You should identify all comments with the docket number 
listed in the notice of availability that publishes in the Federal Register. 
 
For questions regarding this document, contact Melissa Mannion, 301-796-2747. 
 
 
 
 
U.S. Department of Health and Human Services 
Food and Drug Administration 
Center for Drug Evaluation and Research (CDER) 
 
 
August 2020 
Procedural 
 
    
Compliance Policy for the 
Quantity of Bioavailability and 
Bioequivalence Samples 
Retained Under 21 CFR 
320.38(c)  
Guidance for Industry 
 
 
 
 
Additional copies are available from: 
Office of Communications, Division of Drug Information  
Center for Drug Evaluation and Research 
Food and Drug Administration 
10001 New Hampshire Ave., Hillandale Bldg., 4th Floor  
Silver Spring, MD 20993-0002  
Phone: 855-543-3784 or 301-796-3400; Fax: 301-431-6353 
Email: druginfo@fda.hhs.gov  
https://www.fda.gov/Drugs/GuidanceComplianceRegulatoryInformation/Guidances/default.htm 
 
 
 
 
 
 
 
U.S. Department of Health and Human Services 
Food and Drug Administration 
Center for Drug Evaluation and Research (CDER) 
 
 
August 2020 
Procedural
  Contains Nonbinding Recommendations 
 
TABLE OF CONTENTS 
 
 
 
I.  INTRODUCTION............................................................................................................. 1 
II.  BACKGROUND ............................................................................................................... 2 
III.   COMPLIANCE POLICY FOR THE QUANTITY OF SAMPLES RETAINED 
UNDER 21 CFR 320.38(C)........................................................................................................... 3 
APPENDIX 1 ................................................................................................................................. 5 
 
 
  Contains Nonbinding Recommendations 
 
1  Compliance Policy for the Quantity of Bioavailability and 
2  Bioequivalence Samples Retained Under 21 CFR 320.38(c)  
3  Guidance for Industry1 
4   
5   
6  This guidance represents the current thinking of the Food and Drug Administration (FDA or Agency) on 
7  this topic. It does not establish any rights for any person and is not binding on FDA or the public. You can 
8  use an alternative approach if it satisfies the requirements of the applicable statutes and regulations.  To 
9  discuss an alternative approach, contact the FDA office responsible for this guidance as listed on the title 
10  page.   
11   
12   
13   
14   
15  I.  INTRODUCTION  
16   
17  This guidance describes FDAs compliance policy related to the retention of reserve samples of 
18  the test article and reference standard2 used in an in vivo bioavailability (BA) and in vivo or in 
19  vitro bioequivalence (BE) study. Specifically, this guidance: 
20   
21    Addresses the requirement at 21 CFR 320.38(c) to retain reserve samples of sufficient 
22  quantity to permit FDA to perform five times all the release tests required in an 
23  application or supplemental application 
24   
25    Describes the conditions under which the Agency does not generally intend to take 
26  enforcement action against an applicant or contract research organization (CRO) for 
27  retaining less than the quantity of reserve samples of the test article and reference 
28  standard that were used in the BA or BE study as specified in 21 CFR 320.38(c).  
29   
30  This guidance applies only to the requirements for retention of reserve samples contained in 21 
31  CFR 320.38(c). This guidance does not apply to the other requirements for retention of reserve 
32  samples contained in 21 CFR 320.38, such as how testing facilities must select samples for 
33  testing, how the reserve samples must be retained, and whether reserve samples are in fact 
34  representative of the test article and reference standard used in the BA or BE study. Additionally, 
35  this guidance does not apply to the requirement in 21 CFR 211.170 to retain samples under 
36  current good manufacturing practices. 
37   
38   
                                                 
1 This guidance has been prepared by the Office of Generic Drugs, Office of Compliance, and Office of 
Translational Sciences in the Center for Drug Evaluation and Research at the Food and Drug Administration.  
2 A reference standard is the drug product selected by FDA that an applicant seeking approval of an abbreviated 
new drug application must use in conducting an in vivo bioequivalence study required for approval. 21 CFR 
314.3(b).  
1  Contains Nonbinding Recommendations 
 
39  In general, FDAs guidance documents do not establish legally enforceable responsibilities.  
40  Instead, guidances describe the Agencys current thinking on a topic and should be viewed only 
41  as recommendations, unless specific regulatory or statutory requirements are cited. The use of 
42  the word should in Agency guidances means that something is suggested or recommended, but 
43  not required.  
44   
45   
46  II.  BACKGROUND 
47   
48  On November 8, 1990, FDA issued an interim rule that amended, in relevant part, 21 CFR 320 to 
49  require applicants to (1) retain, for a specified period, reserve samples of drug products used to 
50  conduct BA or BE studies and (2) release the reserve samples to FDA when specifically 
51  requested.3 The interim rule was intended to help ensure BE between generic drugs and their 
52  reference listed drug4 and to help FDA investigate possible fraud in BA and BE testing. After 
53  consideration of public comments, a final rule was published in the Federal Register on April 28, 
54  1993.5  
55   
56  This final rule requires a new drug application (NDA) or abbreviated new drug application 
57  (ANDA) applicant to retain a quantity of the test article and reference standard used in BA or BE 
58  testing that is at least five times the amount of product required for release testing.6 Specifically, 
59  21 CFR 320.38 and 320.63 require the applicant (or its CRO) to retain reserve samples of the test 
60  article and reference standard used in conducting any in vivo BA and in vivo or in vitro BE study 
61  that support the approval of its application or supplemental application.  
62   
63  21 CFR 320.38(c) requires that applicants retain reserve samples of the test product and 
64  reference standard used in a BA or BE study that consist of a sufficient quantity to perform five 
65  times all the release tests required in the application or supplemental application.7  Under the 
66  regulations, for solid oral dosage forms (e.g., tablets, capsules), applicants may be required to 
67  retain as many as 300 units each of the test article and reference standard to meet this 
68  requirement.   
69   
                                                 
3 55 FR 47034 (Nov. 8, 1990).  
4 A reference listed drug is the listed drug identified by FDA as the drug product upon which an applicant relies in 
seeking approval of its ANDA. 21 CFR 314.3(b). Generally, the reference listed drug will be the drug product 
selected by the Agency as the reference standard for conducting in vivo BE testing.  
5 58 FR 25918 (Apr. 28, 1993). 
6 21 CFR 320.38(c) and 320.63. 
7 See also guidance for industry Handling and Retention of BA and BE Testing Sample, which discusses 
recommendations for applicants and/or drug manufacturers, CROs, site management organizations, clinical 
investigators, and independent third parties regarding the procedure for handling reserve samples from relevant BA 
and BE studies, as required by 21 CFR 320.38 and 320.63. We update guidances periodically. For the most updated 
version of a guidance, check the FDA guidance web page at https://www.fda.gov/regulatory-information/search-fda-
guidance-documents.  
 
 
2 
  Contains Nonbinding Recommendations 
 
70  Since the final rule was issued in 1993, technological advances in FDAs ability to test these 
71  products have led to test methods that are less destructive and more sensitive, allowing FDA to 
72  detect the identity and composition of the test article and reference standard with smaller 
73  volumes of samples.  As such, under current physicochemical testing, the Agency generally 
74  needs, for products manufactured in single-dose units, 30 units each of the test article and 
75  reference standard from each shipment or, for products manufactured in multi-dose units, 3 units 
76  each of the test article and reference standard from each shipment, to conduct the necessary 
77  testing of the samples.  Consistent with these developments, FDA has received communications 
78  from applicants and CROs requesting to retain a lower quantity of the reserve samples.   
79   
80  In light of these technological advances, FDA finds it appropriate for applicants (or their CROs) 
81  to retain a quantity of samples less than what is specified in 21 CFR 320.38(c) when FDA has 
82  determined that the lower quantity of reserve samples is sufficient for FDA to conduct the 
83  necessary chemical and physical examination of the samples to assure the identity and 
84  composition of the test article and reference standard intended by the regulation.8  Accordingly, 
85  at this time and based on our current understanding of the risks involved, FDA does not intend to 
86  enforce the requirement to retain a sufficient quantity to perform five times all the release tests 
87  required in the application or supplemental application, so long as the identified lower quantities 
88  are retained.  
89   
90   
91  III.   COMPLIANCE POLICY FOR THE QUANTITY OF SAMPLES RETAINED 
92  UNDER 21 CFR 320.38(c) 
93   
94  In light of the technological advances in FDAs ability to test retention samples, FDA does not 
95  intend to take action for violations of 21 CFR 320.38(c) against an NDA or ANDA applicant (or 
96  its CRO) if the quantity of samples retained is sufficient for all Agency testing.9 Appendix 1 of 
97  this guidance provides a list of the minimum sample retention quantities that FDA currently 
98  considers sufficient for all Agency testing for certain products. If a product is included on this 
99  list, FDA does not intend to take action against an applicant for a violation of 21 CFR 320.38(c) 
100  if the applicant retains a quantity of samples consistent with what is indicated on this list. For 
101  products not included in Appendix 1, NDA and ANDA applicants or CROs should submit 
102  requests that FDA not take action if they wish to retain less than the quantity of reserve samples 
103  of the test article and reference standard used in BA or BE testing set forth in 21 CFR 320.38(c). 
104   
105  Requests that are specific to an element of generic drug product development, such as the 
106  retention of BA/BE samples, should be submitted to the Agency as a controlled correspondence. 
107  The guidance for industry Controlled Correspondence Related to Generic Drug Development 
108  discusses the types of requests that would be considered a controlled correspondence, what 
                                                 
8 58 FR 25918, 25923 (Apr. 28, 1993). 
9 Agency testing refers to FDAs chemical and physical examination of the [reserve] samples to assure the identity 
and composition of the test article and reference standard used in a BA or BE study. See 58 FR 25918, 25923 (Apr. 
28, 1993). 
 
 
 
3 
  Contains Nonbinding Recommendations 
 
109  information requestors can include in a controlled correspondence to facilitate FDAs 
110  consideration of and response to a controlled correspondence, and how to submit a controlled 
111  correspondence to FDA.10  
112   
                                                 
10 We update guidances periodically. For the most updated version of a guidance, check the FDA guidance web page 
at https://www.fda.gov/regulatory-information/search-fda-guidance-documents. 
 
 
4 
  Contains Nonbinding Recommendations 
 
113  APPENDIX 1 
114   
115  Minimum Sample Retention Quantities Sufficient for All Agency Testing11 
116   
117  For drug products that are manufactured in single-dose units (i.e., a single unit12 is designed to 
118  provide a single dose of the drug product), FDA does not intend to enforce the requirement to 
119  retain a sufficient quantity to perform five times all the release tests required in the application or 
120  supplemental application, so long as the minimum quantity of drug product for sample retention 
121  is 30 units each of the test article and reference standard from each shipment is retained.13 The 
122  drug products listed in Table 1 are generally manufactured in single-dose units. 
123   
124  Table 1. Drug Products Manufactured in Single-Dose Units 
Route of Administration  Dosage Form 
Buccal  Film 
Oral   Capsule 
Oral  Granule 
Oral  Solution (Single-Dose Container) 
Oral   Tablet 
Oral  Troche/Lozenge 
Transdermal   Film 
Sublingual  Tablet 
Transdermal   Patch 
Urethral  Suppository  
Vaginal  Insert 
Vaginal  Ring 
Vaginal  Suppository  
Vaginal  Tablet 
Intramuscular   Injectable 
Subcutaneous  Injection 
125   
126  For drug products that are manufactured in multi-dose units (i.e., a single unit is designed to 
127  provide more than a single dose of the drug product), FDA does not intend to enforce the 
128  requirement to retain a sufficient quantity to perform five times all the release tests required in 
129  the application or supplemental application, so long as the minimum quantity of drug product for 
130  sample retention is 3 units each of the test article and reference standard from each shipment is 
131  retained.14 The drug products listed in Table 2 are generally manufactured in multi-dose units. 
                                                 
11 This is a list of common drug products of interest and not designed to be an exhaustive list.  
12 The term unit refers to the individual drug product to be dispensed or administered to study subjects.  For 
example, 30 units of an oral tablet means 30 oral tablets.   
13 Requests for a reduction in the quantity of reserve samples for studies involving multiple shipments and study 
sites (e.g., comparative clinical endpoint studies), including any unusual circumstances that may prevent a particular 
study from retaining samples from each shipment, will be addressed on a case-by-case basis.  
14 See footnote 13.  
  5  Contains Nonbinding Recommendations 
 
132  Table 2. Drug Products Manufactured in Multi-Dose Units 
Route of Administration  Dosage Form 
Dental  Powder 
Endocervical  Gel 
Inhalation  Aerosol 
Inhalation  Powder 
Inhalation  Spray 
Intramuscular   Injectable 
Nasal  Aerosol 
Nasal  Spray 
Ophthalmic  Emulsion 
Ophthalmic  Ointment 
Ophthalmic  Solution 
Ophthalmic  Suspension 
Oral  Solution (Multi-Dose Container) 
Oral  Suspension 
Otic  Suspension 
Rectal  Aerosol 
Rectal  Enema 
Rectal  Foam 
Rectal  Gel 
Subcutaneous  Implant 
Subcutaneous  Injection 
Sublingual  Spray 
Topical   Cream 
Topical  Foam 
Topical  Gel 
Topical  Lotion 
Topical  Ointment 
Topical  Shampoo 
Topical   Solution 
Topical  Spray 
Topical  Swab 
Topical  Suspension 
Vaginal  Cream 
Vaginal  Gel 
Vaginal  Ointment 
133   
  6