The recommendation to avoid the use of blood tests for screening are based on the public health guidance Alcohol-use disorders: preventing the development of hazardous and harmful drinking, published by the National Institute for Health and Clinical Excellence (NICE) [NICE, 2010c].

• Both NICE and the Scottish Intercollegiate Guidelines Network (SIGN) [SIGN, 2003] recommend blood tests for monitoring an established alcohol-related problem, but not for routine screening.
• Biochemical markers may lead to false-positive results if used for screening, and are neither sensitive nor specific for diagnosing an alcohol problem [Coulton et al, 2006; Raistrick et al, 2006].
• Gamma-glutamyl transferase (GGT) levels are raised in 60–80% of people with severe alcohol dependence and in 20–50% of heavy drinkers. Usually it takes 4–5 weeks of abstinence from alcohol for GGT levels to return to the normal range. GGT levels may be raised with other causes of liver disease, and less commonly with pancreatic disease or following a heart attack.
• Mean corpuscular volume (MCV) will increase within 4–8 weeks of the onset of heavy drinking. An increase in MCV may also occur in people with vitamin B₁₂ deficiency, folic acid deficiency, thyroid disease, or chronic liver disease, and during pregnancy.
• Carbohydrate deficiency transferrin (CDT) levels will increase within 3–4 weeks of the onset of of heavy drinking, and appear slightly more sensitive than GGT at detecting moderate alcohol misuse (especially in people with liver disease). Nevertheless, this test is not routine practice in primary care, and local policy on its availability will vary. False-positive results for CDT are very unusual.