Reliability of DNA evidence

44.3 The technical reliability of DNA evidence depends on a number of factors, including the quantity and quality of the sample analysed and the laboratory equipment or technique in analysing the sample.

Sample quantity and quality

44.4 A DNA sample is capable of analysis if there is sufficient quantity and reasonable quality of DNA present in the sample. polymerase chain reaction (PCR) based testing is relatively insensitive to degradation. However, the analysis of poor quality DNA samples may lead to uncertain results requiring substantial interpretation by the forensic scientist, and the potential for human error or varying opinions in the interpretation of the results.[3] For example, where a DNA sample contains a mixture of several persons’ DNA, and the forensic scientist does not account for this, the resulting DNA profile may be incorrect.

44.5 In R v Juric, the Victorian Court of Appeal highlighted the difference between evidence produced from a DNA sample that is:

    • so pure and unadulterated that clear typings can be obtained at a large number of DNA sites, giving rise to statistical improbabilities running into the millions or even billions; and

    • so adulterated and so old, and the testing process of amplification so powerful, that the typings produced are affected by complications which preclude an expert from giving an opinion as to the statistical probabilities.[4]

44.6 The Court of Appeal warned that:

there are cases where the simplicity with which the [expert] opinion is expressed cannot be permitted to obscure the difficulties which have been encountered in the testing process. As in this case, those difficulties will include the poverty of the sample, its mixture with the bodily fluids of others, the age of the sample, the effect of the re-amplification process or the reliability of results and whether—because of or in spite of the encountering of these difficulties—any statistical probability can be pronounced as to the likelihood of other members of the community producing the same ‘match’.[5]

Laboratory performance

44.7 The accuracy of DNA analysis depends on the quality control and quality assurance procedures in the forensic laboratory. Quality control refers to measures to help ensure that each DNA analysis result (and its interpretation) meets a required standard of quality. Quality assurance refers to monitoring, verifying and documenting laboratory performance.[6]

44.8 Laboratory accreditation programs provide an important means of ensuring quality control and assurance in the DNA analysis process, by setting minimum standards and procedures, and providing external oversight of adherence to them. The National Association of Testing Authorities, Australia (NATA) operates a national system of laboratory accreditation for forensic science. In Chapter 41, the Inquiry recommended that the Crimes Act 1914 (Cth) (Crimes Act) should provide that forensic analysis of genetic samples must be conducted only by laboratories accredited by NATA in the field of forensic science. This recommendation is equally important as a means of protecting the integrity of DNA analysis and results. However, laboratory accreditation alone cannot guarantee the integrity of DNA evidence in every instance.

Sample handling

44.9 It has been suggested that sample mishandling, mislabelling or contamination is more likely to compromise a DNA analysis than an error in the analysis.[7] Contamination may occur at any stage of the collection, transport or analysis of a DNA sample. A DNA sample may be contaminated with other human DNA in a number of ways, including:

    • the crime scene sample may contain a mixture of fluids or tissues from different persons due to the nature of the crime;

    • the crime scene sample may be contaminated during sample handling at the crime scene or in the laboratory; or

    • carry-over contamination may occur in PCR-based testing if the amplification products of one test are carried over into the mix for a subsequent PCR test.[8]

44.10 One reported example of sample contamination occurred in New Zealand when the DNA profile of an assault victim on the South Island was entered into the DNA data bank and matched the profiles obtained from two separate homicide scenes on the North Island. The DNA samples collected from each crime scene, including the assault, had been analysed in the same forensic laboratory. Police were satisfied the assault victim had not been at either of the homicide scenes at any time, and was not the offender. An independent inquiry could not find any conclusive explanation for the false positive results. The inquiry identified a number of potential sources of contamination, including bench contamination, instrument contamination, failure to observe certain protocols, and deliberate contamination. It concluded that, on the balance of probabilities, the results were caused by accidental contamination of the crime scenes samples during an early stage of processing at the laboratory.[9]

Alternative explanations for a match

44.11 A match between the crime scene profile and a defendant’s profile does not prove that the defendant committed the particular offence. There may be several alternative explanations for a match, including the possibility that laboratory error resulted in a false positive; the sample was ‘planted’ at the crime scene, or was innocently left at the crime scene before, during or immediately after the offence; the sample originated from a close relative of the suspect; or that it originated from an unrelated person who, by coincidence, has the same DNA profile as the suspect.[10]


44.12 Laboratory staff could make errors in conducting DNA analysis, in interpreting or reporting the results of the analysis, or in entering the resulting DNA profile into a DNA database system. This might result from a failure to comply with an established procedure, misjudgement by the scientist, or some other mistake.[11] While protocols and precautions can be introduced to minimise the opportunity for error dur-ing analysis or interpretation, the potential for human error cannot be fully eliminated.

44.13 For example, a clerical error at a Las Vegas forensic laboratory led to an innocent man being charged in relation to two separate sexual assaults in 2001. The man was being held in a detention centre for an immigration law violation when another inmate accused the man of raping him. DNA samples were taken from both men and their profiles were entered into the state DNA database. The man’s profile matched two unsolved sexual assaults, and he was charged with these offences. A DNA expert who examined the laboratory’s records found that the man’s name had been accidentally switched with his cellmate’s name when the profiles were entered into the database, resulting in the false match.[12]

44.14 Misconduct by a forensic scientist could also lead to a false result. In the Queensland case of R v Fitzherbert, the appellant argued that he had been convicted as a result of deliberate fraud on the part of staff at the forensic laboratory that had con-ducted the DNA analysis for the prosecution. The Supreme Court of Queensland dis-missed the appeal on the grounds that there was no evidence to support the allegation.[13]


44.15 Close genetic relatives have more genes in common than unrelated persons.[14] Therefore, it is possible that an innocent person’s DNA profile could match the profile obtained from a crime scene, where the offender was in fact that person’s sibling or other close relative.[15] However, the chance of such a coincidence will decrease inversely as the number of loci examined along the DNA molecule increases.


44.16 A suspect’s DNA profile might match the profile found at a crime scene as a result of tampering with the crime scene, or subsequent substitution of DNA samples. This might occur where the actual offender, a police investigator, or another person deliberately leaves a suspect’s genetic sample at the crime scene. Alternatively, it is possible that a suspect’s sample might later be substituted for the actual crime scene sample to falsely implicate the suspect in the offence.

44.17 In the New South Wales case of R v Lisoff, the defendant alleged that DNA evidence implicating him in an assault had been planted on his clothes by police investigators after they took them into custody. The defence expert witness suggested that the blood found on the clothes appeared to be post-transfusion blood from the victim, which must have been deposited on the clothing after it was taken into police custody. The victim’s blood sample had been stored in the same police exhibit room as the accused’s clothing.[16]

44.18 While practices and procedures for the collection of crime scene samples, and the handling of those samples during transfer to the laboratory, and at the laboratory itself, may seek to minimise the opportunity for tampering, it cannot be eliminated altogether.


44.19 As a DNA profile contains only a very small section of a person’s DNA, it is possible that two persons might have the same DNA profile, by coincidence. This is particularly the case where the profile represents only a small number of loci along the DNA molecule. A widely reported example of a coincidental match occurred in Britain in 1999. A man was charged with burglary as a result of a ‘cold hit’ between his DNA profile and a crime scene profile on the United Kingdom’s national DNA database. The profiles matched at six loci along the DNA molecule, but there was no match upon subsequent comparison at ten loci. The match probability had been reported as one in 37 million.[17]

44.20 At the time this incident was reported, the custodian of the national DNA database admitted that, in light of the number of profiles then stored on the database, testing at six loci would produce several hundred chance matches.[18]

[3]D Kaye and G Sensabaugh Jr, ‘Reference Guide on DNA Evidence’ in Federal Judicial Center (ed), Reference Manual on Scientific Evidence (2000) Washington DC, 485, 505–506, 508.

[4]R v Juric (Unreported, Supreme Court of Victoria, Court of Appeal, Winneke P; Charles and Chernov JJA, 29 May 2002) [20].

[5]Ibid [20].

[6]D Kaye and G Sensabaugh Jr, ‘Reference Guide on DNA Evidence’ in Federal Judicial Center (ed), Reference Manual on Scientific Evidence (2000) Washington DC, 485, 509.

[7]Ibid, 512.

[8]Ibid, 514–515.

[9]Rt Hon Sir Thomas Eichelbaum and Sir John Scott, Report on DNA Anomalies (1999), Auckland. The report commented that while there was no direct evidence of contamination, they had eliminated all other hypotheses: [8.3].

[10] See generally, D Kaye and G Sensabaugh Jr, ‘Reference Guide on DNA Evidence’ in Federal Judicial Center (ed), Reference Manual on Scientific Evidence (2000) Washington DC, 485, 520. See also Legislative Council Standing Committee on Law and Justice, Review of the Crimes (Forensic Procedures) Act 2000, Report No 18 (2002), Parliament of NSW, Sydney [3.54].

[11]D Kaye and G Sensabaugh Jr, ‘Reference Guide on DNA Evidence’ in Federal Judicial Center (ed), Reference Manual on Scientific Evidence (2000) Washington DC, 485, 510.

[12]G Puit, ‘DNA Evidence: Officials Admit Error, Dismiss Case’, Las Vegas Review-Journal, 18 April 2002.

[13]R v Fitzherbert (Unreported, Supreme Court of Queensland, Pincus, Davies JJA and Moynihan J, 30 June 2000).

[14]D Kaye and G Sensabaugh Jr, ‘Reference Guide on DNA Evidence’ in Federal Judicial Center (ed), Reference Manual on Scientific Evidence (2000) Washington DC, 485, 522–523.

[15] For example, see R v Watters, (Unreported, Court of Appeal Criminal Division, Kay LJ; Silber J; Mellor HHJ, 19 October 2000).

[16]R v Lisoff (Unreported, NSW Court of Criminal Appeal, Spigelman CJ, Newman and Sully JJ, 22 November 1999) [56]. See also G Urbas, ‘DNA Evidence in Criminal Appeals and Post-Conviction Inquiries: Are New Forms of Review Required?’ (2002) 2 Macquarie Law Journal 141, 156.

[17]L Lee, ‘England Man to Sue Police Over DNA Mistake’, Newsbytes (Minneapolis), 18 February 2000. The man had an alibi, lived 200 miles from the crime scene, and was suffering from Parkinson’s disease. There was no other evidence linking him to the crime.

[18] See M Goode, ‘Some Observations on Evidence of DNA Frequency’ (2002) 23 Adelaide Law Review 45, 57.