Best Practice Recommendations for Internal Validation of Human 

Short Tandem Repeat Profiling on Capillary Electrophoresis 

Platforms

Best Practice 

Recommendations for 

Internal Validation of 

Human Short Tandem 

Repeat Profiling on 

Capillary Electrophoresis 

Platforms

Biological Methods Subcommittee

Biology/DNA Scientific Area Committee

Organization of Scientific Area Committees (OSAC) for Forensic Science
Best Practice Recommendations for Internal Validation of Human 

Short Tandem Repeat Profiling on Capillary Electrophoresis 

Platforms

OSAC Proposed Standard

Best Practice Recommendations for 

Internal Validation of Human Short 

Tandem Repeat Profiling on Capillary 

Electrophoresis Platforms

Prepared by 

Biological Methods Subcommittee

Version: 1.0

Disclaimer:

This document has been developed by the Biological Methods Subcommittee of the Organization of 

Scientific Area Committees (OSAC) for Forensic Science through a consensus process and is 

proposed for further development through a Standard Developing Organization (SDO). This document 

is being made available so that the forensic science community and interested parties can consider the 

recommendations of the OSAC pertaining to applicable forensic science practices. The document was 

developed with input from experts in a broad array of forensic science disciplines as well as scientific 

research, measurement science, statistics, law, and policy. 

This document has not been published by an SDO. Its contents are subject to change during the 

standards development process. All interested groups or individuals are strongly encouraged to submit 

comments on this proposed document during the open comment period administered by the Academy 

Standards Board (www.asbstandardsboard.org). 
Best Practice Recommendations for Internal Validation of Human 

Short Tandem Repeat Profiling on Capillary Electrophoresis 

Platforms

Foreword

This document outlines best practice recommendations for the internal validation of human short 

tandem repeat DNA profiling on capillary electrophoresis platforms utilized in forensic 

laboratories. This standard will provide guidance for laboratories when conducting internal 

validations.

This standard was revised, prepared and finalized as a standard by the DNA Consensus Body of the 

AAFS Standards Board (ASB). The initial draft document was developed by the Biological Methods 

Subcommittee of the Organization of Scientific Area Committees. All hyperlinks and web addresses 

shown in this document are current as of the publication date of this standard.

Keywords: validation, internal validation, short tandem repeats, capillary electrophoresis
Best Practice Recommendations for Internal Validation of Human 

Short Tandem Repeat Profiling on Capillary Electrophoresis 

Platforms

Table of Contents

1 Scope..................................................................................................................................................................... 1

2 Normative References.................................................................................................................................... 1

3 Terms and Definitions.................................................................................................................................... 1

4 Requirements.................................................................................................................................................... 3

4.1 General Considerations for Validation Set-up ............................................................................ 3

5 Best Practice Recommendations for Internal Validation Standards for STR Profiling Using 

CE Platforms ............................................................................................................................................................4

5.1 Sensitivity...................................................................................................................................................4

5.2 Analytical Threshold ..............................................................................................................................5

5.3 Peak Height Ratio ...................................................................................................................................6

5.4 Stutter ......................................................................................................................................................... 7

5.5 Stochastic Threshold.............................................................................................................................8

5.6 Mixtures .....................................................................................................................................................9

5.7 Precision................................................................................................................................................. 10

5.8 Contamination ......................................................................................................................................11

5.9 Concordance..........................................................................................................................................13

5.10 Knowns and Non-Probative.......................................................................................................... 13

Annex A ................................................................................................................................................................ 15

Annex B .................................................................................................................................................................16
1

Best Practice Recommendations for Internal Validation of Human 

Short Tandem Repeat Profiling on Capillary Electrophoresis 

Platforms

1 Scope

This document provides best practice recommendations for performing an internal validation of a 

human short tandem repeat (STR) multiplex kit using capillary electrophoresis (CE). It is a 

companion document to “Standard for Internal Validation of Human STR Profiling on Capillary 

Electrophoresis Platforms”.

2 Normative References

2.1 Standards for Internal Validation of Forensic DNA Analysis Methods (ASB Standard 38)

2.2 Standards for Internal Validation of Human Short Tandem Repeat Profiling on Capillary 

Electrophoresis Platforms (ASB Standard 39)

3 Terms and Definitions

For the purpose of this document the following definitions apply:

3.1

analytical threshold

The minimum height requirement at and above which detected peaks on a STR DNA profile 

electropherogram can be reliably distinguished from instrument background noise; peaks above 

this threshold are generally not considered noise and are either artifacts or true alleles.

3.2

artifact

A non-allelic product of the amplification process (e.g., stutter, non-templated nucleotide addition, 

or other non-specific product), an anomaly of the detection process (e.g., single or multi-channel 

voltage spikes or “pull-up”), or a by-product of primer synthesis (e.g., “dye blob”) that may be 

observed on an electropherogram.

3.3

capillary electrophoresis (CE)

An electrophoretic technique for separating DNA molecules by their relative size based on their 

differential migration through a capillary tube filled with a liquid polymer.

3.4

controls

Samples of known types, run in parallel with experimental, reference, or evidence samples that are 

used to demonstrate that a procedure is working correctly.

3.5

drop-in

Allelic peak(s) in an electropherogram that are not reproducible across multiple independent 

amplification events.

3.6

drop-out

Failure of an otherwise amplifiable allele to produce a signal above analytical threshold because the 

allele was not present or was not present in sufficient quantity in the aliquot that underwent PCR 

amplification.
2

Best Practice Recommendations for Internal Validation of Human 

Short Tandem Repeat Profiling on Capillary Electrophoresis 

Platforms

3.7

internal validation

The accumulation of test data within the laboratory for developing the laboratory standard 

operating procedures and demonstrating that the established protocols for the technical steps of 

the test and for data interpretation perform as expected in the laboratory.

3.8

locus (loci)

A unique physical location of a gene (or specific sequence of DNA) on a chromosome.

3.9

peak height ratio

The relative ratio of two peaks at a given locus.

3.10

polymerase chain reaction (PCR)

An enzymatic process by which a specific region of DNA is replicated during repetitive cycles that 

consist of the following: denaturation of the template; annealing of primers to complementary 

sequences at an empirically determined temperature; and extension of the bound primers by a DNA 

polymerase.

3.11

precision studies

Studies performed to evaluate the degree of mutual agreement among a series of individual 

measurements, values and/or results.

3.12

probabilistic genotyping

The use of biological modeling (i.e., statistical modeling informed by biological data), statistical 

theory, computer algorithms, and/or probability distributions to infer genotypes and/or calculate 

likelihood ratios.

3.13

sensitivity studies

A set of critical studies performed during developmental and/or internal validation of DNA or other 

test methods designed to define the lower and upper limits/bounds of an assay to accurately detect 

an analyte.

3.14

short tandem repeats (STR)

Multiple copies of an identical (or similar) DNA sequence arranged in direct succession where the 

repeat sequence unit is 2 base pairs (bp) to 6 bp in length.

3.15

stochastic threshold

The peak height value in a DNA electrophoretic profile above which it is reasonable to assume that, 

at a given locus, allelic drop-out of a sister allele in a heterozygous pair has not occurred in a single 

source DNA sample.
3

Best Practice Recommendations for Internal Validation of Human 

Short Tandem Repeat Profiling on Capillary Electrophoresis 

Platforms

3.16

Stutter

An artifact of PCR amplification typically observed one or more repeat units smaller or larger than 

an STR allele in a DNA electrophoretic profile, may result from strand slippage during PCR 

amplification. A stutter peak is generally of lower RFU than the allele peak.

3.17

validation

The process of performing and evaluating a set of experiments that establish the efficacy, reliability, 

and limitations of a method, procedure or modification thereof; establishing recorded 

documentation that provides a high degree of assurance that a specific process will consistently 

produce an outcome meeting its predetermined specifications and quality attributes. May include 

developmental and/or internal validation.

4. Requirements

4.1 General Considerations for Validation Set-up

4.1.1 Samples should be recently quantified using current laboratory procedures.

4.1.2 Applicable controls (e.g. positive, negative, reagent blank) should be run with all sample 

sets throughout the validation process.

4.1.3 Single-source DNA samples with a high degree of heterozygous genotypes from multiple 

donors should be used (1).

4.1.4 If a laboratory uses measures to enhance detection sensitivity (e.g. increased 

amplification cycle number, increased injection time, and post-amplification 

purification), additional studies to determine the effect of these measures on 

interpretation shall be performed.

4.1.5 Re-analysis of validation data shall be performed if alterations are made to the data 

analysis software parameters which impact sizing, peak height, or peak detection (e.g. 

smoothing, peak half-widths, sizing algorithm selection).

4.1.6 Outliers or discrepancies identified during data analysis should be further evaluated, 

and may dictate the need for additional testing or the implementation of more 

conservative thresholds.

4.1.7 Testing for normality during data analysis can increase the laboratory’s confidence in

conclusions drawn from the data set (e.g. peak height ratio threshold).

4.1.8 Calibration and/or maintenance status of the laboratory instrumentation used should 

be verified prior to sample analysis to ensure the systems are operating optimally and 

within manufacturer’s parameters.

4.1.9 An assessment within six months of casework implementation should be performed by 

the laboratory to evaluate the effectiveness of the validated parameters, and if 

necessary, create additional data that may further improve process workflow, 

thresholds, and interpretations.

5 Best Practice Recommendations for Internal Validation Standards for STR
4

Best Practice Recommendations for Internal Validation of Human 

Short Tandem Repeat Profiling on Capillary Electrophoresis 

Platforms

Profiling Using CE Platforms

5.1 Sensitivity

5.1.1 Standard

The laboratory shall determine the sensitivity level and optimal genomic DNA input range for 

each set of STR test kit conditions/parameters (e.g. PCR cycle number, injection time, voltage, 

PCR reaction volume, etc.) with the new STR test kit.

5.1.2 Objective

The purposes of these studies are to define the upper and lower limits for the STR test kit and 

capillary electrophoresis platform used, and to identify an optimal DNA template target or 

target range for developing interpretable DNA profiles.

5.1.3 Considerations

It may be informative to perform replicates of the entire experimental setup with an emphasis 

on additional replicates in the stochastic range of the STR test kit.

5.1.4 Experimental Method

5.1.4.1 Single-source DNA samples processed in-house using internally validated 

methods should be used for this study. Purchased human DNA may be used for 

comparison, or to supplement laboratory processed samples.

5.1.4.2 DNA templates known to have greater separation (i.e. multiple repeats apart) 

between alleles for heterozygous genotypes should be included to account for variation in 

PCR efficiency due to fragment size.

5.1.4.3 Serial dilutions of a minimum of three unique DNA samples should be performed, 

with the range of DNA template quantities higher and lower than expected to be processed 

by the laboratory.

5.1.4.3.1 Dilutions at lower concentrations should produce STR profiles where allele 

dropout is observed.

5.1.4.3.2 The serial dilution should contain a minimum of five different template 

quantities performed in triplicate (e.g. three unique DNA samples x five dilutions x three 

replicates = 45 total amplifications).

5.1.5 Data Analysis and Results

5.1.5.1. Variation at different DNA template quantities should be characterized using 

average peak height (APH), standard deviation, and coefficient of variance for every locus at 

each DNA template quantity for both homozygotes and heterozygotes.

5.1.5.2 Upper and lower limits of reliable interpretation along with optimal DNA input 

target/range should be defined.

5.1.5.2.1 Limits can be evaluated using RFU and DNA templatequantity.

5.1.5.2.2 Upper limits should be informed by evaluation of spectral pull-up, excessive 

stutter, off scale signal, increased artifact detection, and decreased locus and allelic 

balance.

5.1.5.2.3 Lower limits should be informed by, peak height balance, allelic and locus 

drop-out, allelic drop-in, and elevated stutter.

5.1.5.2.4 Optimal input DNA can be a target or a range and will fall within the highest 

and lowest concentration of DNA template from a single contributor that clearly
5

Best Practice Recommendations for Internal Validation of Human 

Short Tandem Repeat Profiling on Capillary Electrophoresis 

Platforms

distinguishes true alleles (homo- and heterozygous) from artifacts, demonstrates intraand inter-locus and channel balance, and usually results in complete profiles.

5.1.6 Implementation

5.1.6.1 Data obtained from the sensitivity study can form the basis for the laboratory’s

policies on:

5.1.6.1.1 Optimal amplification setup: 

5.1.6.1.1.1 Single-source target input range 

5.1.6.1.1.2 Positive control DNA input target 

5.1.6.1.1.3 Stochastic threshold, if applicable

5.1.6.1.1.4 Upper limit of analysis, that prevents an increased observation of offscale/artifact peaks

5.1.6.1.1.5 Lower limit of analysis, that informs processing and interpretation 

strategies

5.1.6.1.2 Expectations on locus balance and peak heights of pristine samples:

5.1.6.1.2.1 Assist with troubleshooting of caseworksamples

5.1.6.1.2.2 Identification of possible mixturessamples

5.2 Analytical Threshold

5.2.1 Standard

The laboratory shall determine the analytical threshold for each dye channel of the STR test kit 

using a range of sample types and DNA template input quantities across multiple analyses.

5.2.2 Objective

The purpose of setting an analytical threshold (AT) is to define the RFU value above which 

artifacts and true allelic signal are differentiated from baseline noise.

5.2.3 Considerations

5.2.3.1 CE instruments should be set on a solid/secure platform that protects against 

potential vibration or movement that could affect baseline noise.

5.2.3.2 The laboratory should use a set of samples with DNA templates representative of 

those typically analyzed in casework.

5.2.3.3 Caution should be taken when utilizing data higher than the laboratories 

determined optimal DNA input range since this may increase the calculated AT resulting in 

the potential loss of interpretable data.

5.2.4 Experimental Method

5.2.4.1 Amplification negatives and samples using a range of DNA templates generated 

during validation may be used.

5.2.4.2 Non-baseline peaks (e.g. dye-artifacts, other known artifacts, spectral pull-up, and 

allelic peaks) should be removed from data prior to calculation.

5.2.4.3 Samples displaying off-scale data or excessive artifacts should not be used.

5.2.5 Data Analysis and Results

5.2.5.1 Analysis should be performed by examining signal from each dye channel.

5.2.5.2 Multiple methods for calculating an AT are acceptable, and examples can be found in 

the literature (2).

5.2.6 Implementation
6

Best Practice Recommendations for Internal Validation of Human 

Short Tandem Repeat Profiling on Capillary Electrophoresis 

Platforms

Analytical thresholds should be established early in the validation process and applied to all 

other validation studies.

5.3 Peak Height Ratio

5.3.1 Standard

The laboratory shall characterize expected peak height ratio variation observed for each locus 

of the STR test kit utilizing single-source samples amplified over a range of DNA input amounts.

5.3.2 Objective

Characterizing peak height ratios (PHR) during internal validation provides the laboratory with 

data to assist in understanding allelic imbalance, identifying the possibility of allelic dropout, 

and the development of interpretation guidelines.

5.3.3 Considerations

5.3.3.1 Measurements of peak area may be substituted for peak height.

5.3.3.2 Laboratories performing manual mixture deconvolution should evaluate the effects 

of mixture ratios, the number of contributors, and DNA concentration on PHR.

5.3.3.3 Laboratories employing probabilistic genotyping software for mixture 

deconvolution should characterize PHRs to assist with preliminary profile interpretation 

prior to software deconvolution.

5.3.3.3.1 Laboratories should consult with software manufacturer and user manual to 

ensure the proper experimental design is used for probabilistic genotyping software 

validation.

5.3.3.4 Off-scale data should not be used to measure PHR since the peak height data is not 

accurate for off-scale peaks.

5.3.3.5 Laboratories should calculate PHRs using alleles from heterozygous pairs separated 

by two or more repeats to avoid contribution from stutter artifacts.

5.3.4 Experimental Method

5.3.4.1 See Sensitivity Study Experimental Method.

5.3.5 Data Analysis and Results

5.3.5.1 PHR for each heterozygous pair of alleles should be calculated (e.g. low RFU 

peak/high RFU peak)

5.3.5.2 Average PHR, standard deviation, minimum and maximum PHR should be 

calculated.

5.3.5.3 PHR variation should be evaluated using RFU data for one or more of the following 

approaches:

5.3.5.3.1 The full range of data generated.

5.3.5.3.2 Separation based upon RFU (e.g. low RFU vs. high RFU).

5.3.5.3.3 Separation based on locus.

5.3.5.4 PHR variation should be characterized to determine the minimum value(s) to be 

used in profile interpretation and under defined conditions (e.g. template quantity, peak 

heights). This can be accomplished using various methods (e.g. using the average PHR 

minus three standard deviations).

5.3.5.4.1 The laboratory should document the number of events that fall below the 

minimum threshold(s).
7

Best Practice Recommendations for Internal Validation of Human 

Short Tandem Repeat Profiling on Capillary Electrophoresis 

Platforms

5.3.5.5 The laboratory should create a plot of peak height and/or input value vs. PHR to 

determine data linearity.

5.3.6 Implementation

5.3.6.1 The results from this study can form the basis for the laboratory’s minimum PHR

expectations for assistance in data interpretation.

5.3.6.2 The following PHR threshold approaches may be implemented based on laboratory 

requirements:

5.3.6.3 A single universal minimal expected PHR threshold.

5.3.6.4 Multiple locus-specific or RFU-specific minimal expected PHR thresholds.

5.4 Stutter

5.4.1 Standard

The laboratory shall characterize expected PCR stutter artifacts observed for each STR locus of 

the STR test kit.

5.4.2 Objective

Characterizing stutter ratios during internal validation provides the laboratory with data to 

ensure the STR kit displays stutter frequencies comparable to that reported during 

developmental validation and can assist with the development of interpretation guidelines.

5.4.3 Considerations

5.4.3.1 Laboratories utilizing the manufacturer’s recommendations should compare 

internally validated stutter data to manufacturer's developmentally validated data to 

ensure consistency.

5.4.3.1.1 The laboratory should define the level of acceptable variation from 

developmentally validated values.

5.4.3.2 Laboratories employing probabilistic genotyping software for mixture 

deconvolution should characterize stutter to assist with preliminary profile interpretation 

prior to software processing.

5.4.3.3 Laboratories should use single-source samples processed within the span of the 

optimal DNA input range, at a minimum, to evaluate PCR stutter artifacts.

5.4.3.4 Off-scale data should not be used to measure stutter since the peak height data is 

not accurate for the off-scale peak.

5.4.4 Data Analysis and Results

5.4.4.1 Stutter peaks should be characterized based on size and amplitude relative to an 

allelic peak. (e.g. = (RFU of stutter artifact)/ (RFU of allele peak))

5.4.4.2 Stutter characterization should be classified into categories based on relation to the 

true allele peak:

5.4.4.2.1 Two repeat units smaller

5.4.4.2.2 One repeat unit smaller

5.4.4.2.3 One repeat unit larger

5.4.4.2.4 Partial or other repeat unit differences

5.4.4.3 The data analysis software employed should have all stutter filters removed and the 

allele-calling threshold set to a value low enough to capture all non-template peaks (e.g. 20 

RFU).
8

Best Practice Recommendations for Internal Validation of Human 

Short Tandem Repeat Profiling on Capillary Electrophoresis 

Platforms

5.4.4.3.1 Non-stutter artifacts must be edited out prior to data export, such as spectral 

pull-up, incomplete adenylation, CE spikes or dye artifacts.

5.4.4.4 Stutter should not be calculated for the following conditions:

5.4.4.4.1 When two alleles at an individual locus are one repeat unit different in size, as 

stutter contribution cannot be decoupled from the allele height.

5.4.4.4.2 When two alleles are two repeat units apart, as the n+1 stutter from the first 

allele is additive with the n-1 stutter of the second allele.

5.4.4.5 Allele designation, base pair size and peak height data must be exported for accurate 

stutter analysis.

5.4.4.6 At a minimum, the following characteristics for stutter should be calculated using 

the above data:

5.4.4.6.1 Average stutter per locus

5.4.4.6.2 Standard deviation

5.4.4.6.3 Minimum and maximum stutter observed at each locus.

5.4.4.7 A test of the success of the stutter thresholds should be performed using either 

single-source mock casework or non-probative samples to ensure that the majority of 

stutter peaks are filtered out.

5.4.5 Implementation

5.4.5.1 The laboratory may determine a stutter threshold using one of the following 

methods:

5.4.5.1.1 Average plus a determined number of standard deviations

5.4.5.1.2 Maximum observed per locus

5.4.5.2 If fewer than five observations occurred at a particular locus, it is acceptable to use 

the largest observed stutter value.

5.4.5.3 Each laboratory should decide if they wish to implement stutter thresholds only for 

the most commonly observed (e.g. n-1 repeat unit) stutter in the data analysis software 

used by the laboratory or to implement additional stutter thresholds (e.g. +1, n-2 repeat 

unit).

5.4.5.3.1 At a minimum, the less common stutter values should be made available for 

staff to consider during interpretation.

5.5 Stochastic Threshold

5.5.1 Standard

The laboratory shall determine a stochastic threshold (ST) for any binary interpretation 

method used to interpret the STR test kit data or employ a probabilistic genotyping method.

5.5.2 Objective

Identification of an ST allows the laboratory to determine the peak height value above which it 

is reasonable to assume that, at a given locus, allelic dropout of a sister allele in a heterozygous 

pair has not occurred (3).

5.5.3 Considerations

5.5.3.1 Data to assess ST may be generated using the sensitivity samples.

5.5.3.1.1 If additional samples are needed beyond the sensitivity study, these should be 

selected from below the optimal DNA template range.
9

Best Practice Recommendations for Internal Validation of Human 

Short Tandem Repeat Profiling on Capillary Electrophoresis 

Platforms

5.5.3.2 Stochastic effects within an amplification reaction may affect one or more loci 

irrespective of locus or allele size.

5.5.3.3 The following are contributing factors when establishing a stochastic threshold:

5.5.3.3.1 Amplification conditions (e.g. DNA template input, reaction volume, or cycle 

number)

5.5.3.3.2 CE parameters (e.g. injection time or voltage).

5.5.3.4 Stochastic threshold should be calculated for all amplification conditions and 

capillary electrophoresis parameters the laboratory plans to use

5.5.4 Experimental Method

5.5.4.1 See Sensitivity Study Experimental Method.

5.5.5 Data Analysis and Results

5.5.5.1 Derive a stochastic threshold using DNA template inputs that exhibit allele drop-out 

such that the sister allele is not visible (true drop-out) and/or drop-out where the sister 

allele is below the previously established analytical threshold (straddle drop-out).

5.5.5.2 The stochastic thresholds may be defined using:

5.5.5.2.1 Average peak height plus a determined number of standard deviations

5.5.5.2.2 Logistic regression

5.5.5.2.3 Value above highest peak height where most severe imbalance is observed

5.5.5.2.4 Plotting PHR vs average RFU

5.5.5.3 All loci/dye channels should be assessed; however, a single threshold may be 

implemented.

5.5.6 Implementation

5.5.6.1 The derived stochastic threshold should be applied to case-type samples and the 

performance evaluated.

5.5.6.2 If the threshold is too high, true homozygotes are flagged as possible allelicdropout.

5.5.6.3 If the threshold is too low, heterozygotes with allelic dropout would not be flagged.

5.5.6.2 If the stochastic threshold does not filter the majority of the data accurately, then 

additional samples may need to be incorporated in the data analysis.

5.6 Mixtures

5.6.1 Standard

The laboratory shall conduct a study utilizing mixed DNA samples that are representative of the 

number of contributors, DNA ratios, and genomic DNA template quantities expected to be 

interpreted by the testing laboratory.

5.6.2 Objective

These studies should assist the laboratory in establishing guidelines for mixture interpretation; 

this may include an estimate of the number of contributors to the mixture, determination of the 

major and minor contributor profiles, and contributor ratios.

5.6.3 Considerations

5.6.3.1 This study should be performed after the sensitivity, peak height ratio, analytical 

threshold, stochastic, and stutter studies have been completed and thresholds have been 

established.

5.6.3.2 The laboratory should design the study to incorporate one additional contributor
10

Best Practice Recommendations for Internal Validation of Human 

Short Tandem Repeat Profiling on Capillary Electrophoresis 

Platforms

beyond the number expected to be interpreted during casework in order to demonstrate 

the ability to assess the number of contributors and the limitations of the laboratory’s 

mixture interpretation process.

5.6.3.3 To further develop interpretation guidelines, a study of the number of contributors 

paired with multiple different genomic template quantities and qualities (e.g. degraded, 

inhibited) should be performed.

5.6.3.4 If a laboratory has validated or is currently planning the validation of a Y-STR test 

kit, then studies should be planned for efficient test design and workflow decisions which 

can be performed in parallel.

5.6.3.4.1 Decisions about downstream STR typing may be impacted by the 

autosomal/Y-STR mixture ratios and upstream quantification values.

5.6.4 Experimental Method

5.6.4.1 Mixture series should be created by generating a specified range of ratios from well 

characterized single-source samples.

5.6.4.1.1 The range of ratios should include minor alleles that fall below stochastic and 

analytical thresholds.

5.6.4.1.2 The range of ratios should be designed in order to identify the limits of 

major/minor determination.

5.6.4.1.3 Samples with a range of allele variants should be included to evaluate 

performance and prevent excessive allelic overlap and masking.

5.6.4.2 A minimum of two mixture series should be performed in duplicate at ratios 

representative of those expected to be interpreted by the laboratory.

5.6.5 Data Analysis and Results

5.6.5.1 Laboratories should apply filters and thresholds developed in the other studies (e.g.

analytical threshold, stochastic threshold (if applicable), peak height ratio, stutter ratios) on 

data produced for the mixture study.

5.6.5.2 An evaluation of each mixture ratio of the series should be performed to determine 

the ratio at which alleles from the minor contributor fall below established thresholds.

5.6.5.3 Alleles that exhibit masking should not be employed for a manual mixture ratio 

estimation.

5.6.5.4 The observed mixture ratio should be assessed for all samples and instances in 

which the major and minor contributors become indistinguishable should beidentified.

5.6.6 Implementation

5.6.6.1 Standards for implementation are to be issued in a separate document. (4)

5.7 Precision

5.7.1 Standard

The laboratory shall demonstrate allelic sizing precision and calling accuracy of the new STR 

test kit and associated capillary electrophoresis instrument through repeatability and 

reproducibility studies.

5.7.2 Objective

The purpose of evaluating precision for base pair sizing is to assess the analytical performance 

of the separation mechanism and software sizing of DNA fragments included in the STR typing
11

Best Practice Recommendations for Internal Validation of Human 

Short Tandem Repeat Profiling on Capillary Electrophoresis 

Platforms

test kit.

5.7.3 Considerations

5.7.3.2Environmental factors may need to be considered prior to placement ofCE 

instrument or adjustments may need to be made to room conditions.

5.7.3.3 Base pair sizing variation greater than 0.5 base pairs may result in incorrect allele 

calls.

5.7.4 Experimental Method

5.7.4.1 Ambient room temperature and humidity should be monitored during instrument 

runs.

5.7.4.2 Base pair sizing precision measurements should be performed using injections of 

allelic ladders in different capillaries throughout the course of the validationStudies.

5.7.4.3 A base pair sizing precision study should be performed for each CE instrument in 

use in the laboratory/laboratories.

5.7.4.4 A base pair sizing precision study should be performed for each instrument 

parameter/run condition that may affect migration (e.g. run voltage, polymer, etc.).

5.7.4.5 Allelic ladders should be evaluated to allow for the characterization of variation 

across capillaries, injections, and time.

5.7.5 Data Analysis and Results

5.7.5.1 Calculate the range of maximum and minimum allele sizes across all allelic ladders 

to assist with the identification of possible microvariants or run-to-runvariation.

5.7.5.2 The precision for base pair sizing is measured by using the base pair size for each 

allele in the allelic ladder generated from the analysis software.

5.7.5.3 The average base pair size of each allele is calculated using these data and the 

standard deviation calculated. Three times standard deviation (a confidence interval of 

99.73%) provides precision estimates for each allele of a locus and should be less than 0.5 

base pairs.

5.7.5.4 If precision is determined to be greater than 0.5 base pairs for any allele or locus, 

sources of the cause of this deviation should be examined (e.g. environmental conditions, 

run parameters, etc.). Once the source of the deviation is remedied, precision should be 

reassessed.

5.7.6 Implementation

5.7.6.1 Results should allow the laboratory to determine the frequency of ladder injections 

and to identify if re-injections related to off ladder allele designations are necessary.

5.7.6.2 Environmental factors may need to be considered for placement of CE instrument or 

adjustments to room conditions.

5.8 Contamination

5.8.1 Standard

The laboratory shall determine the susceptibility of the genotyping process to the introduction 

and detection of exogenous human DNA through the evaluation of controls (e.g. reagent blanks, 

negative and positive controls) and samples with known genotypes.

5.8.2 Objective

The purpose of the contamination study is to inform the laboratory as to the susceptibility of
12

Best Practice Recommendations for Internal Validation of Human 

Short Tandem Repeat Profiling on Capillary Electrophoresis 

Platforms

the STR analysis system to the detection of exogenous DNA originating from reagents, 

consumables, laboratory operator, instrument, and environment.

5.8.3 Considerations

The following criteria should be considered for the origin of contamination (6):

5.8.3.1 Two categories of exogenous DNA should be evaluated. Contamination where an 

unexpected source of DNA is detected and allele drop-in where an unexpected allele 

(possibly two) are detected.

5.8.3.2 Laboratory environment (e.g. possible new cleaning schedule/procedure, HVAC 

systems, workflow/dedicated lab space, etc.)

5.8.3.3 Consumables (e.g. tubes, reagents, pipette tips, etc.)

5.8.3.4 Operator (e.g. appropriate training, PPE)

5.8.3.5 Capillary Electrophoresis instrument (e.g. capillary injection carry-over, capillary 

cross-talk)

5.8.3.6 Automation equipment (e.g. normalization, amplification setup, post-amplification 

loading).

5.8.4 Experimental Method

5.8.4.1 No additional studies are necessary for the contamination assessment. Data 

generated from the other validation studies should be used for the contaminationstudy.

5.8.4.2 Additional controls may be processed to test the system based on the variables 

described in the considerations above.

5.8.5 Data Analysis and Results

5.8.5.1 All samples, including controls, should be evaluated for the presence of exogenous 

DNA (including allele drop-in) which may originate from reagents, consumables, operator 

and/or laboratory environment.

5.8.5.2 Allele drop-in should be documented and a frequency of occurrence should be 

determined.

5.8.5.3 Drop-in rate may be estimated by comparing the number of drop-in observances to 

the total amount of data or samples evaluated during the validation study.

5.8.5.4 Contaminating allelic data, if present, should be characterized and source 

attribution attempted. The source of the contaminating data (co-processed samples, 

laboratory operator, consumables etc.) may identify the point in the laboratory process that 

the contamination event occurred and inform the laboratory on how to adjust proceduresto 

prevent recurrence, identify potential systemic problems that may require further 

improvements, and direct contamination tolerance interpretation guidelines.

5.8.6 Implementation

The results from this study can form the basis for the laboratory’s policies on:

5.8.6.1 Appropriate procedure setup (including placement and number of controls)

5.8.6.2 Laboratory environment (design, workflow, cleanup/maintenance)

5.8.6.3 Level of tolerance (drop in frequency/expectations)

5.8.6.4 Contamination management and necessary corrective measures

5.8.6.5 Control measures (e.g. Personal Protective Equipment)

5.9 Concordance
13

Best Practice Recommendations for Internal Validation of Human 

Short Tandem Repeat Profiling on Capillary Electrophoresis 

Platforms

5.9.1 Standard

The laboratory shall conduct an STR genotyping concordance study.

5.9.2 Objective

The purpose of concordance testing is to demonstrate agreement between STR typing results 

obtained compared to those using previous methods or published data (5).

5.9.3 Considerations

5.9.3.1 Concordance samples should be evaluated after conditions for casework sample 

analysis have been established (e.g. target DNA amount, injection time and voltage, PCR 

cycles/volume).

5.9.3.2 The samples used in the concordance study should reflect the type of STR test kit 

being validated (e.g. autosomal STR typing test kit should use both male and female DNA, 

direct amplification typing test kits should use buccal swabs, or stain punches; Y-STR test 

typing kit should use male DNA).

5.9.4 Experimental Method

5.9.4.1 The sample types used in the concordance study must include the appropriate NIST 

Standard Reference Material (SRM) and may include proficiency test samples, amplification 

positive controls, purchased blood samples, or genomic DNA derived from stable celllines.

5.9.5 Data Analysis and Results

5.9.5.1 A comparison of observed allele to the known values (if established) should be 

performed.

5.9.5.2 The absence of known allele calls at a locus should be documented.

5.9.5.3 Observed discordance should be documented and if possible, a reason provided.

5.9.6 Implementation

5.9.6.1 Observed discordance may not invalidate the concordance study. Common reasons 

for discordance may include a null allele resulting from a primer binding site mutation, a 

difference in allele call due to different PCR primer sets, or a different method of fragment 

separation affecting resolution or migration.

5.10 Knowns and Non-Probative

5.10.1 Standard

The laboratory shall evaluate the new STR test kit using the laboratory testing procedures, and 

samples representative of those regularly analyzed by the testing laboratory.

5.10.2 Objective

The purpose of the knowns and non-probative study is to assess the performance of validated 

parameters of the STR test kit using case-type samples processed using upstream laboratory 

procedures (7).

5.10.3 Considerations

5.10.3.1 This study may be used to assess relative STR test kit performance through 

comparison with the previously validated STR kit data.

5.10.3.2 The new STR test kit being validated should be robust and generate reproducible 

results for pristine samples and consistent results for challenged samples.

5.10.3.3 Laboratories should determine if validated extraction chemistry procedure(s) 

introduce inhibitors that interfere with the amplification of DNA with the STR test kit.
14

Best Practice Recommendations for Internal Validation of Human 

Short Tandem Repeat Profiling on Capillary Electrophoresis 

Platforms

5.10.4 Experimental Method

5.10.4.1 The testing laboratory should analyze known and case-type samples representing 

those expected to be encountered by the laboratory for casework using the new STR test kit. 

Some sample type considerations include:

5.10.4.1.1 Known samples should be single-source DNA samples of good quality and 

not limited in concentration.

5.10.4.1.2 Different genomic template quantities including limited DNA samples.

5.10.4.1.3 Inhibited samples.

5.10.4.1.4 Adulterated samples (e.g. latent print processing reagents, gun oil, etc.).

5.10.4.1.5 Samples containing more than one contributor in varying template amounts.

5.10.4.1.6 Degraded samples, including differential degradation in mixed samples.

5.10.4.1.7 All extraction methods currently in use by the testing laboratory should be 

represented in the above samples.

5.10.5 Data Analysis and Results

5.10.5.1 Data should be evaluated using parameters established during internal validation 

of the STR kit.

5.10.5.2 STR typing results should be compared to any previous results whereapplicable.

5.10.5.3 STR typing results should be compared to reference DNA profiles, if available, to 

determine the ability to include or eliminate individuals.

5.10.5.4 Results from the known and case-type samples should be evaluated to determine if 

any adverse effect is observed that may be attributable to extraction chemistry (e.g. signal 

reduction, partial or complete inhibition, peak height imbalance, locus imbalance, 

preferential amplification, incomplete adenylation or other artifacts).

5.10.6 Implementation

5.10.6.1 When evaluating known and non-probative samples, if parameters developed 

during internal validation of the STR test kit fail to produce the expected outcome, then 

parameter(s) should be re-evaluated prior to implementation
15

Best Practice Recommendations for Internal Validation of Human 

Short Tandem Repeat Profiling on Capillary Electrophoresis 

Platforms

Annex A

(informative)

Bibliography

The following information provides a list of the literature resources: 

1] Masters JR, et al. 2001. PNAS 98(14):8012-17.

2] Bregu et al., Analytical thresholds and sensitivity: establishing RFUthresholds for 

forensic DNA analysis. J Forensic Sci. 2013 58:120-9.

3] Bieber et al. BMC Genetics (2016) 17:125.

4] ANSI/ASB Standard 020, Standard for Validation Studies of DNA Mixtures and 

Development and Verification of a Laboratories Mixture Interpretation Protocol,first 

edition, 2018

5] ISO18385:2016MinimizingtheriskofhumanDNAcontaminationinproductsusedtocollect,store 

and analyzebiological material for forensicpurposes – Requirements. It is available at:

https://www.iso.org/standard/62341.html

6] Butler JM. Quality Assurance and Validation. In: Advanced Topics in Forensic DNA 

Methodology. Elsevier, 2011.

7] Bright JA, et al. 2011. J Forensic Sci. 2011 Jan;56(1):181-5.
16

Best Practice Recommendations for Internal Validation of Human 

Short Tandem Repeat Profiling on Capillary Electrophoresis 

Platforms

Annex B

(informative)

Tables and Figures

Cross function study - This table is intended to assist laboratories with efficiently utilizing 

sample data generated across the multiple studies outlined in this document. The x-axis is 

a category list of the samples in process and the y-axis is the listed outline of each study 

described in this document. The precision study has ladder samples generated across all 

runs.