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Genetic Testing - The Quality And Utility Of Genetic Tests

disease positive negative people

Like all diagnostic and screening tests, the quality and utility of genetic tests depend on their reliability, validity, sensitivity, specificity, positive predictive value, and negative predictive value. Reliability of testing refers to the test's ability to be repeated and to produce TABLE 6.1
DNA-based gene tests available, 2004
SOURCE: "Some Currently Available DNA-based Gene Tests," in Gene Testing U.S. Department of Energy, Office of Biological and EnvironmentalResearch, Human Genome Project, http://www.ornl.gov/sci/techresources/Human_Genome/medicine/genetest.shtml (accessed February 17, 2005)

Alpha-1-antitrypsin deficiency (AAT; emphysema and liver disease)
Amyotrophic lateral sclerosis (ALS; Lou Gehrig's Disease; progressive motor function loss leading to paralysis and death)
Alzheimer's disease* (APOE; late-onset variety of senile dementia)
Ataxia telangiectasia (AT; progressive brain disorder resulting in loss of muscle control and cancers)
Gaucher disease (GD; enlarged liver and spleen, bone degeneration)
Inherited breast and ovarian cancer* (BRCA 1 and 2; early-onset tumors of breasts and ovaries)
Hereditary nonpolyposis colon cancer* (CA; early-onset tumors of colon and sometimes other organs)
Charcot-Marie-Tooth (CMT; loss of feeling in ends of limbs)
Congenital adrenal hyperplasia (CAH; hormone deficiency; ambiguous genitalia and male pseudohermaphroditism)
Cystic fibrosis (CF; disease of lung and pancreas resulting in thick mucous accumulations and chronic infections)
Duchenne muscular dystrophy/Becker muscular dystrophy (DMD; severe to mild muscle wasting, deterioration, weakness)
Dystonia (DYT; muscle rigidity, repetitive twisting movements)
Fanconi anemia, group C (FA; anemia, leukemia, skeletal deformities)
Factor V-Leiden (FVL; blood-clotting disorder)
Fragile X syndrome (FRAX; leading cause of inherited mental retardation)
Hemophilia A and B (HEMA and HEMB; bleeding disorders)
Hereditary Hemochromatosis (HFE; excess iron storage disorder)
Huntington's disease (HD; usually midlife onset; progressive, lethal, degenerative neurological disease)
Myotonic dystrophy (MD; progressive muscle weakness; most common form of adult muscular dystrophy)
Neurofibromatosis type 1 (NF1; multiple benign nervous system tumors that can be disfiguring; cancers)
Phenylketonuria (PKU; progressive mental retardation due to missing enzyme; correctable by diet)
Adult Polycystic Kidney Disease (APKD; kidney failure and liver disease)
Prader Willi/Angelman syndromes (PW/A; decreased motor skills, cognitive impairment, early death)
Sickle cell disease (SS; blood cell disorder; chronic pain and infections)
Spinocerebellar ataxia, type 1 (SCA1; involuntary muscle movements, reflex disorders, explosive speech)
Spinal muscular atrophy (SMA; severe, usually lethal progressive muscle-wasting disorder in children)
Thalassemias (THAL; anemias—reduced red blood cell levels)
Tay-Sachs Disease (TS; fatal neurological disease of early childhood; seizures, paralysis) [3/99]
Note: Susceptibility tests, noted by an asterisk, provide only an estimated risk for developing the disorder.

equivalent results in comparable circumstances. A reliable test is consistent and measures the same way each time it is used with the same patients in the same circumstances. For example, a well-calibrated balance scale is a reliable instrument for measuring body weight.

Validity is the accuracy of the test. It is the degree to which the test correctly identifies the presence of disease, blood level, or other quality or characteristic it is intended to detect. For example, if you put an object you knew weighed ten pounds on a scale and the scale said it weighed ten pounds, then the scales results are valid. There are two components of validity: sensitivity and specificity.

Sensitivity is the test's ability to identify people who have the disease. Mathematically speaking, it is the percentage of people with the disease who test positive for the disease. Specificity is the test's ability to identify people who do not have the disease—it is the percentage of people without the disease who test negative for the disease. Ideally, diagnostic and screening tests should be highly sensitive and highly specific, thereby accurately classifying all people tested as either positive or negative. In practice, however, sensitivity and specificity are frequently inversely related—most tests with high levels of sensitivity have low specificity, and the reverse is also true.

The likelihood that a test result will be incorrect can be gauged based on the sensitivity and specificity of the test. For example, if a test's sensitivity is 95%, then when one hundred patients with the disease are tested, ninety-five will test positive and five will test "false negative"—they have the disease but the test has failed to detect it. For example, disorders such as Charcot-Marie-Tooth disease can arise from mutations in one of many different genes, and since some of these genes have not yet been identified, they will not be detected and a false negative result might be reported. On the other hand, if a test is 90% specific, when one hundred healthy, disease-free people are tested, ninety will receive negative test results and ten will be given "false-positive" results, meaning that they do not have the disease but the test has inaccurately classified them as positive.

The positive predictive value is the percentage of people that actually have the disease of all those with positive test results. The negative predictive value measures the percent of all the people with negative test results who do not have the disease.

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