Multiplex STR Kits, Inhibitor Handling and Low-Template DNA

A PCR inhibitor is any substance that reduces the efficiency of the amplification reaction below the threshold where all loci amplify at expected peak heights. Inhibitors reach forensic extracts because they are co-purified with DNA from the original evidence matrix. Four inhibitor classes are of operational significance.

Haematin is the most common and most studied. It is released from haemoglobin by alkaline hydrolysis or prolonged enzymatic degradation and appears in blood stains, blood-saturated soil, and decomposed biological material. Haematin's mechanism is dual: it chelates the Mg2+ ions that Taq polymerase requires for catalytic activity, and it directly inhibits Taq by binding to the enzyme active site. As little as 20 µM haematin in a 25 µL PCR reaction produces significant inhibition. Haematin inhibition is the mechanism that makes blood-stained soil a particularly challenging extraction substrate, a situation encountered in mass grave investigations (such as the 2004 tsunami recovery operations in Thailand and Indonesia, and the post-conflict exhumations at ICMP operations in Bosnia), buried homicide victims (as studied in ENFSI working group reports on challenging samples), and crime scenes where decomposition has been prolonged.

Humic acid and fulvic acids are degradation products of plant organic matter, present in soil, peat, and surface runoff. They are co-extracted with DNA from soil swabs and environmental samples. Humic acids inhibit Taq by intercalating into double-stranded DNA (blocking polymerase progression) and by interacting with the negatively charged backbone of nucleic acids in ways that disrupt primer annealing. The concentration at which humic acids produce detectable inhibition in standard PCR is approximately 1 µg/mL.

Indigo is released from denim fabric into body fluid stains deposited on jeans or indigo-dyed fabrics. The criminal forensic significance is obvious: sexual assault and violent crime evidence frequently involves denim. Indigo operates by intercalating into double-stranded template and primer-template duplexes, producing preferential drop-out of larger STR amplicons. The 2002 study by Benecke and Schmitt in the International Journal of Legal Medicine established indigo as a significant inhibitor in German casework.

Tannic acids, present in leather goods, plant material, and heavily tanned textiles, inhibit Taq by a protein-binding mechanism at concentrations as low as 0.05 µg/mL. They are relevant in cases involving leather gloves, belts, or vehicle interiors with leather upholstery.

The Internal PCR Control (IPC) spike is the primary inhibition-detection tool in modern forensic kits. Investigator 24plex QS, InnoQuant HY (in the quantitation step), and several other kits include a non-human, non-forensic DNA sequence that is spiked into the reaction at a fixed concentration. If the IPC amplification Ct shifts upward relative to the expected value, inhibition is present and its severity can be estimated. A shift of 2 to 3 Ct cycles indicates moderate inhibition; a shift of more than 5 cycles indicates severe inhibition requiring remediation before any result from the plate can be trusted.

Dilution is the simplest remediation. If an extract is sufficiently concentrated, diluting 1:2 to 1:10 reduces the inhibitor concentration below the threshold for inhibition while maintaining an amplifiable amount of template DNA. The Quantifiler Trio result guides this decision: a sample quantified at 2 ng/µL with an inhibition flag can be diluted to 0.5 ng/µL (reducing both template and inhibitor by 4-fold) and re-amplified. Dilution is ineffective when the extract is already at a low concentration.

Centrifugal ultrafiltration through devices such as the Amicon Ultra (Merck Millipore) or the MinElute PCR Purification Kit (Qiagen) physically retains high-molecular-weight DNA on the membrane while allowing smaller inhibitor molecules to pass through in the filtrate. This works for low-molecular-weight inhibitors (indigo, some inorganic salts) but not for inhibitors of similar or higher molecular weight than DNA (proteins, humic acids can partially co-elute).

Chelex 100 chelation, an ion-exchange resin, binds divalent cations including the iron released from haematin, reducing haematin's Mg2+-chelating mechanism. Chelex extraction is therefore specifically useful for blood-stained soils and decomposed biological matrices where haematin is the dominant inhibitor.

BSA (bovine serum albumin) is a protein additive used in some PCR protocols at 0.1 to 0.8 mg/mL. BSA binds to polyphenolic inhibitors (humic acids, tannic acids) and to haematin, sequestering them and preventing their interaction with Taq. Several UK and German SOPs for challenging samples include BSA as a direct supplement to the amplification reaction.

The Chelex DNA IQ System (Promega) and the PrepFiler BTA Forensic DNA Extraction Kit (Applied Biosystems) incorporate inhibitor-removal chemistry into the extraction step itself, reducing post-extraction inhibitor carry-over. These extraction platforms are specified for challenging sample types in the validated SOPs of several US state crime laboratories and Australian state forensic laboratories.

Low-template DNA (LT-DNA) amplification refers to any workflow where the input template is below approximately 100 to 200 pg per reaction, the range at which stochastic sampling effects become significant. With so few template molecules, random variation in whether a particular allele molecule is captured in a given reaction aliquot means the two alleles of a heterozygous genotype may amplify to very different peak heights (heterozygote imbalance), or one allele may fail to amplify entirely (allelic drop-out), or a contaminant DNA molecule may amplify to a detectable level (allelic drop-in).

Low-copy-number (LCN) PCR, as developed at the UK Forensic Science Service in the late 1990s and described by Gill, Whitaker, Sparkes, and colleagues in 1997 in Electrophoresis, increases the cycle number from 28 to 34 cycles. This 6-cycle increase raises theoretical amplification from ~268 million to ~17 billion copies, generating profiles from template masses where standard PCR would fail. The stochastic artefacts are not eliminated; they are the expected consequence of the approach and must be interpreted using probabilistic methods.

In 2007, the Sean Hoey trial in Northern Ireland (R v. Hoey [2007] NICC 49) became the first major UK case where LCN DNA evidence was subjected to sustained judicial scrutiny. Mr Justice Weir excluded the LCN evidence, not because the chemistry was unsound, but because the UK FSS had not implemented protocols for dealing with background contamination at the sensitivity of LCN, and the interpretation framework was insufficiently documented to allow independent scrutiny. The UK Forensic Regulator's office subsequently commissioned a review (the Caddy Report, 2008), which found LCN valid but required enhanced contamination controls and probabilistic genotyping for mixture interpretation before LCN evidence could be used in UK courts.

Equivalent debates followed in the United States (the New York City Office of the Chief Medical Examiner was the first major US laboratory to adopt enhanced-cycle-number PCR for casework, triggering a 2011-2016 inter-laboratory validation study published in Forensic Science International: Genetics) and in Australia (the Victorian Institute of Forensic Medicine published validation data for a 30-cycle enhanced protocol in 2013).

In India, the CFSL laboratories have not widely adopted LCN workflows for casework; the NABL DNA testing criteria and the proposed DNA Technology Bill implementation framework do not yet specify LCN-specific validation requirements, though several academic publications from the CFSL Chandigarh and DFSS Gujarat have described enhanced-cycle amplification for challenging samples.

The operational response to a low-template sample at an accredited forensic laboratory involves three interacting decisions: which amplification protocol, which kit, and how to handle the resulting electropherogram data.

Increased cycle number is the first and most accessible adjustment. Standard kits specify 28 to 29 cycles. For template inputs between 50 and 200 pg, validated SOPs at many laboratories specify 30 to 32 cycles. For inputs below 50 pg, 34 cycles. Each additional cycle doubles the theoretical amplicon yield from any template molecule that is amplifying, including contaminant molecules. This is why enhanced contamination controls (doubled negative controls, pre-extraction UV decontamination, enhanced PPE, blanked consumable lots) are mandatory alongside any increased cycle protocol.

The Investigator 24plex GO! (Qiagen) is a kit formulated for the low-template range: it uses an optimised buffer, higher cycle number in the default protocol (34 cycles), and a shorter annealing time to favour correct-template amplification over artefact generation. AmpFlSTR MiniFiler (Applied Biosystems) and the Investigator Decaplex (Qiagen) use mini-STR primer design (amplicons below 200 bp) to maximise locus recovery from fragmented DNA where standard 400 bp amplicons would fail. These kits are widely used in cold-case re-examination, skeletal identification, and challenging contact-trace samples.

Post-PCR clean-up removes residual primers, primer dimers, and unused dNTPs from the amplification product before CE injection. At standard template inputs these are handled by the Hi-Di formamide denaturation step; at low template inputs, primer dimer peaks can obscure the small allele peaks produced by low-copy amplification. ExoSAP-IT (Applied Biosystems), which combines Exonuclease I (degrades single-stranded primers) and Shrimp Alkaline Phosphatase (dephosphorylates free dNTPs), is used in some validated low-template SOPs to clean the amplified product before CE injection.

Probabilistic genotyping is the analytical partner to low-template amplification. Software such as STRmix (developed by ESR New Zealand and the FSANZ validation consortium), TrueAllele (Cybergenetics), and EuroForMix (Oslo University Hospital) models the stochastic effects of low-template amplification probabilistically and computes a likelihood ratio that accounts for allelic drop-out and drop-in rather than forcing a binary present/absent allele call. Without probabilistic genotyping, a low-template partial profile is very difficult to interpret correctly in a mixture context.

In practice, the quantitation result from Section 4 of this module drives the kit and protocol selection decision described here. A high-quality extract at 0.5 to 2 ng/µL with no inhibition flag and a Degradation Index below 3 routes directly to the standard 28-cycle protocol of the laboratory's primary kit (GlobalFiler, PowerPlex Fusion 6C, Investigator 24plex QS, depending on jurisdiction and database alignment). The result can be submitted to CODIS, NDNAD, or the relevant national database under the standard analytical thresholds.

A quantitation result of 0.05 to 0.5 ng/µL with a DI below 3 and no inhibition routes to an increased-cycle protocol (30 to 32 cycles) with the same primary kit. Results are interpreted under a validated stochastic threshold.

A quantitation result with significant inhibition flags routes to dilution or re-extraction before any amplification decision. The analyst should not proceed directly to LT-PCR on an inhibited sample; the inhibition will compound the stochastic effects and produce an uninterpretable result.

A low DI but very low yield (below 50 pg) routes to a validated LT-DNA protocol: increased cycles (32 to 34), enhanced contamination controls, and probabilistic genotyping for result interpretation. The SWGDAM Low Template DNA Amplification guidelines (2010), the UK FSR Codes of Practice Annex D low template provisions (2023), and the ENFSI DNA WG Best Practice Manual for LT-DNA (2017) all govern this pathway.

A high DI combined with low yield routes to a mini-STR kit (Investigator 24plex GO!, AmpFlSTR MiniFiler) to maximise locus recovery from fragmented template, followed by probabilistic genotyping.

1. Review quantitation result
   Check autosomal ng/µL, Y fraction if relevant, Degradation Index, and inhibition flags from Quantifiler Trio, Plexor HY, or InnoQuant HY.
2. Select kit and protocol
   High quality, no inhibition: primary kit (GlobalFiler / PowerPlex Fusion 6C / Investigator 24plex QS), 28-29 cycles. Inhibition present: dilute or re-extract first. Low yield, low DI: increase cycle number. High DI: mini-STR kit.
3. Apply contamination controls
   Double negative controls for any LT-DNA run. Verify consumable lot. UV decontaminate workspace. Document all lot numbers.
4. Amplify
   ProFlex Thermal Cycler, Veriti, or GeneAmp 9700 on the validated programme. Document instrument ID, program name, start time, analyst ID.
5. CE injection
   Prepare CE injection plate in the post-PCR zone. Denature in Hi-Di formamide. Apply post-PCR clean-up (ExoSAP-IT) if validated for LT-DNA protocol.
6. Interpret
   Apply validated analytical threshold (AT). Assign alleles. For LT-DNA or mixture: apply probabilistic genotyping (STRmix, TrueAllele, EuroForMix). Calculate LR against database/reference.