How do we know what the standards of practice are for any given scientific task?
Every scientific discipline has its own set of specific requirements. “Consensus standards” are the objective written requirements for specific scientific disciplines. They are meant to represent the minimum objective requirements for performing work in a particular scientific field. For an opinion to be considered reliable, the requirements of the consensus standard(s) must be objectively satisfied. The minimum requirements of metrology are found in the ISO 17025 consensus standard. Therefore, for any blood alcohol measurement, the prosecution must prove that it has complied with the minimum objective requirements of ISO 17025.
The Minimum Requirements of a Measurement Found in the ISO 17025 Standard
The generally accepted consensus standard for any measurement is found in ISO 17025. ISO stands for the International Organization for Standardization. The ISO is an international organization whose members are authorities on specific disciplines of science. The ISO does not accredit or certify scientific work. Rather they publish standards, which are used to guide how scientific work is conducted and evaluated. These standards are based upon the generally accepted opinions of the scientific community.
The standard does not set lofty and idealized goals. On the contrary, it is the floor for scientific reliability. It is intended to set minimum requirements. As forensic toxicologist Chester Flaxmayer states the standard is meant to represent general requirements of what the majority of scientists believe should be done. In the case of ISO 17025, the standard is a set of minimum requirements for producing reliable measurements. Failure to comply with the requirements of ISO 17025 is prima facie evidence that a measurement is not trustworthy.
Some of the fundamental provisions of ISO 17025 include: (1) Section 5.5.7, which discusses when measuring equipment must be taken out of service; and (2) Section 4.11.2, which provides when and why a root cause analysis for malfunctioning equipment is required. These two requirements are not optional; and a measurement that fails to comply with them risks exclusion under a Daubert analysis.
Accreditation Does Not Mean Reliability
On its face, the idea of a crime laboratory having an outside agency perform some sort of peer review seems like a sound quality measure. Unfortunately, accreditation through the American Association of Crime Lab Directors (ASCLD) is often mischaracterized as a type of independent peer review, guaranteeing the trustworthiness of scientific work. 
During a trial involving a laboratory with ASCLD LAB accreditation, inevitably the State will bring out this assertion to infer a guarantee of the reliability of their scientific evidence. However, what is really being done is the fallacy of an “argument from authority.” An “argument from authority” is a logical fallacy where source of the assertion is the primary basis for its belief. It plays on a psychological phenomenon where there is a stronger likelihood of agreement with a false conclusion if the source of the knowledge is believed to have a high-status.
There is significant evidence that ASCLD has not earned such status. ASCLD is a not a government regulatory agency. It is a form of self-regulation or in some cases merely the appearance of self-regulation. Under ASCLD, members of crime labs assist in accrediting each other (for a fee). In practice, the primary criteria for accreditation is making sure a laboratory has reliable written standards in place. However, they do not ensure that a lab actually follows the standard (i.e. ISO 17025).
When inspections are conducted a laboratory is given prior notice of when it will occur. That is, accreditation is not based upon a blind inspection. An example of an inspection, for lab’s blood alcohol section, would include informing the section of the inspection. While a laboratory may conduct thousands of blood alcohol tests in a given year, only five (5) cases are required to be reviewed by ASCLD LAB. Just as significant, the crime laboratory gets to select which test results are going to be reviewed by the inspectors.