Laboratory Testing Methods

#Begin with what the method measures

A laboratory method is designed to measure or detect something particular. A quantitative result expresses an amount in units; a qualitative result describes a category such as detected or not detected. CDC’s testing guide distinguishes these forms of output. Neither form is a complete diagnosis by itself.

An immunoassay uses antibody-related recognition of a target. MedlinePlus describes ELISA as an enzyme-linked immunoassay in which a targeted antibody or antigen is linked to an enzyme and a signal, such as a colour change, supports detection. What that signal means depends on the target and the specific test. The method’s name alone does not establish which health question it can answer.

Evidence: CDC: Ready? Set? Test! quality practices / MedlinePlus: ELISA blood test / MedlinePlus: understanding lab results

#Distinguish growth from genetic detection

A bacteria culture gives organisms in a sample conditions in which to grow so they can be examined and identified. MedlinePlus explains that additional susceptibility testing can assess responses to antibiotic medicines. This is a description of the laboratory method, not advice about which medicine to use or how to grow a sample.

PCR detects selected genetic material by making many copies of a target sequence. Tests for an RNA target can involve conversion to DNA before amplification. MedlinePlus notes that PCR can sometimes find pieces of a virus after infection has ended. Detection of genetic material is therefore not automatically proof of an organism’s current activity or the cause of symptoms. Culture and PCR answer different measurement questions.

Evidence: MedlinePlus: bacteria culture tests / MedlinePlus: PCR tests

#Read cell measurements as part of the picture

A complete blood count measures several features, including blood cell counts, hemoglobin, hematocrit and average red-cell size. A CBC with differential adds information about the types of white blood cells. These are related measurements, not one all-purpose test for every illness.

MedlinePlus explains that abnormal values do not always mean a condition needs treatment and that history, symptoms and other findings help establish a diagnosis. A count can be useful without explaining the cause of a change. This page provides no normal values, diagnostic cutoffs, interpretation of an instrument flag or advice to start or stop testing.

Evidence: MedlinePlus: complete blood count / MedlinePlus: understanding lab results

#Keep technical performance and clinical usefulness separate

Quality control assesses aspects of whether a test system is working as expected. CDC’s guidance also considers identification, documentation and the surrounding workflow. An apparently precise instrument result can still be unhelpful if the wrong material was tested or the result is used outside its supported purpose.

MedlinePlus notes that laboratories may use different methods and reference ranges, so values are not automatically interchangeable. FDA’s diagnostic-test guidance also distinguishes accuracy against a reference standard from agreement with another method. A result needs evidence suited to the clinical question as well as a sound measuring process. This is general method education, not a laboratory procedure, test recommendation or Mynd testing service.

Evidence: CDC: Ready? Set? Test! quality practices / MedlinePlus: understanding lab results / FDA: reporting diagnostic test performance

Source note

The sections above were checked against the linked sources. No clinical review has been performed. This is general research education, not a clinical guideline.