A chemistry screen is a blood test done to check for normal levels of various blood elements. A chemistry screen measures levels of the following blood parameters: electrolytes, specific proteins, lipids, sugar, enzymes associated with specific organs, blood gases, waste products, and other blood elements.
There are many different reasons a physician may order a blood chemistry screen. A chemistry screen may be done as part of a routine examination to assess normal body function or a routine blood laboratory workup prior to surgery. Chemistry screens are used to identify potential disease states present in a patient, to monitor the progression of a patient’s disease, to monitor disease treatment or recurrence of a disease, to observe levels of certain prescription medicines, or to assess the effects of prescription medicines that may be harmful.
Chemistry screens are performed whenever medically necessary regardless of age, gender, or race. They are routinely done on patients before surgical procedures.
Chemistry screens provide information on the quantity of specific chemical parameters in the blood. The patient’s blood levels are listed along with a reference range of values for each component. The reference range indicates what the normal range of values is, from low to high. The chemistry screen compares the patient’s blood levels to the reference range, and flags the patient’s blood values as falling outside of the normal range when necessary. If the patient’s values fall within the reference range, the value is considered normal. If the patient’s value is higher or lower than the reference range, the physician then evaluates the results and follows a proper course of action. Whether or not a patient’s blood chemistry screen demonstrates normal results can help determine whether a patient has a condition requiring surgery, or whether they are fit enough for a particular surgical procedure to be successful.
Chemistry screens can test for many different blood parameters, and can be customized by the physician to fit a particular patient’s medical needs. When chemistry screens are customized, the physician merely indicates on the prescription that parameters not usually included in a standardized screen are to be included. Standardized forms of chemistry screens often used in the hospital include the Chem-6, Chem-7, Chem-12, and Chem-20, which measure 6, 7, 12, and 20 different blood components, respectively. The type of chemistry screen chosen by the physician is determined by the reason for the chemistry screen, any diseases the patient has, and specific medical symptoms the patient is experiencing that need to be explored via the results of the screen. The Chem-20 is the most thorough, and includes all the parameters measured in the smaller screens with some additions.
The Chem-20 Chemistry Screen
The Chem-20 chemistry screen is also known as a Sequential Multi-channel Analysis -20 (SMA-20) screen. The Chem-20 tests for 20 different chemical parameters listed below.
Blood Parameters Measured in the Chem-20 Chemistry Screen
- alanine aminotransferase (ALT);
- alkaline phosphatase (ALP);
- aspartate aminotransferase (AST);
- blood urea nitrogen (BUN);
- carbon dioxide;
- conjugated bilirubin;
- gamma glutamyl transpeptidase (GGT);
- lactate dehydrogenase (LDH);
- total bilirubin;
- total cholesterol; and
- uric acid.
The blood components measured in the chemistry screen may be affected by various disease or nutritional states. A physician can learn much about the health condition of a patient by interpreting the results of a chemistry screen. The overall picture of health presented by the combined measurements in the chemistry screen are more informative than any one value alone.
General Description of Blood Parameters Measured in a Chemistry Screen
Albumin is a blood component made in the liver. Albumin binds to and carries certain substances in the blood, including some medications. It is important for keeping the proper amount of fluid within blood vessels, tissue growth, and healing. Albumin is measured to help assess liver and kidney function, as well as nutritional state.
NORMAL RESULTS REFERENCE RANGE (MAY VARY SLIGHTLY BY TESTING LABORATORY)
- alanine aminotransferase: 4-36 U/L;
- albumin: 3.5-5.5 g/dl;
- alkaline phosphatase: males 38-126 U/L, females 70-230 U/L;
- aspartate aminotransferase: 8-35 U/L;
- bicarbonate: 21-27 mEq/L;
- blood urea nitrogen: 7-18 mg/dl;
- calcium: 8.4-10.2 mg/dl;
- carbon dioxide: 35-45 mm Hg;
- chloride: 98-106 mEq/L;
- conjugated bilirubin: 0-0.2 mg/dl;
- creatinine: 0.6-1.2 mg/dl;
- gamma glutamyl transpeptidase: 7-50 U/L;
- glucose: 70-115 mg/dl;
- lactate dehydrogenase: 90-190 U/L;
- phosphate: 2.7-4.5 mg/dl; males >60 years old 2.3-3.7, females 2.8-4.1 mg/dL;
- potassium: 3.5-5.1 mEq/L;
- sodium: 135-145 mEq/L;
- total bilirubin: 0.2-1.0 mg/dl;
- total cholesterol: <200 mg/dl; and
- uric acid: males 3.5-7.2 mg/dl, females 2.6-6.0 mg/dl.
Blood urea nitrogen tests how well the kidneys are functioning to remove waste from the body for excretion in the urine. Creatinine is another parameter measured to help assess kidney function. The ratio of the amount of BUN and creatinine present in the blood provides a more detailed picture of kidney function than either measurement alone. Uric acid is an indicator of kidney function in removing waste from the blood.
ALT, ALP, AST, GGT, and LDH are all enzymes associated with the liver whose levels help assess liver function and whether the liver is damaged. In addition to the liver, ALP is associated with the kidney, bones, and placenta. LDH is also associated with many different organs including the heart, brain, and skeletal muscle and is released with tissue damage. Bilirubin is a breakdown product of red blood cells that is taken up from the blood by the liver, altered, and secreted through the bile into the digestive tract where it is partially excreted in feces. Measurements involving
QUESTIONS TO ASK YOUR DOCTOR
- Why do I need a chemistry screen?
- Is the screen being done to look for signs of a specific disease?
- Do I need to fast before the chemistry screen?
- Will any of my prescription or non-prescription medications, herbal, or nutritional supplements affect the results of the chemistry screen?
- When will I get the results of the screen?
- Could any of the results affect my surgery if they are abnormal?
bilirubin assess how well the liver and associated biliary tract is functioning.
Electrolytes such as potassium, sodium, and chloride are measured in chemistry screens. Electrolytes are minerals found naturally in the body; are necessary to keep a balance in body fluids; to maintain normal body functions such as heart rhythm, muscle contraction, and brain function. Electrolyte imbalances can be caused by various disease states, including kidney disease. Calcium and phosphate levels may also be affected by kidney function, parathyroid disorders, and certain bone diseases. Both chloride and bicarbonate levels are indicative of acid base disorders and the ability of the blood to buffer acid and base to determine the pH value.
Cholesterol is measured to assess the level of fatty substances in the blood that affect the arteries and the heart. Glucose levels are potential indicators of liver function, pancreatic function, and the body’s ability to utilize sugar for energy. Carbon dioxide gas levels in the blood assess the function of both the lungs and the kidneys. Each component of the chemistry screen can be affected in distinct ways by many different disease states. Interpreting the results of the chemistry screen requires much training and experience.
Chemistry screens are done using blood samples. Having blood drawn from a vein with a syringe, usually in the arm, is necessary. Some parameters of the chemistry screen may require a period of fasting from all food and drink (except water) before the test. Patients should also avoid high fat foods or alcohol the night before the test. Since some medications may affect the results of the chemistry screen, it is critical
Bile— Green fluid secreted by the liver, stored in the gallbladder, and released into the digestive tract. Bile aids in the digestion of fat.
Biliary Tract— The passages that allow the flow of bile from the gallbladder to the intestines.
Parathyroid Gland— Small endocrine glands that release hormones to regulate calcium and phosphorous.
pH— A value that indicates the balance between acid and base in a given substance.
Phlebitis— Inflammation of a vein.
that the physician take into account all prescription medications, non-prescription medications, herbal, and nutritional supplements that the patient is taking before running the chemistry screen. Some people may have slightly high or low values as their normal level. Age and gender may also affect the results in predictable patterns.
There is very little risk associated with having blood drawn for a chemistry screen. Most people have no side effects; some may get a small bruise where the syringe was inserted. With any blood draw there is a small chance that the area around the punctured vein may develop phlebitis, the inflammation of a vein. Phlebitis may also involve a bacterial infection if the site of the blood draw was not appropriately cleaned before the needle was inserted. Phlebitis can be locally painful but usually resolves in a short period of time.
Additionally, patients with disorders involving the inability of the blood to form normal blood clots should discuss their condition and their medications with the physician before the blood draw and chemistry screen is done.
A chemistry screen is prescribed by a physician. It is a routine test that is run before a surgical procedure. A nurse often draws the blood sample from the patient. The blood sample is then sent to a specific hospital laboratory that tests the blood. The results of the chemistry screen are then sent to the physician for review.
Chaudhry, H. J., et al. Fundamentals of Clincal Medicine, 4th ed. Philadelphia: Lippincott Williams & Wilkins, 2004.
Maxwell, R. W. Maxwell Quick Medical Reference, 5th ed. Tulsa, OK: Maxwell Publishing Company, 2006.
“Chemistry Screen.” WebMD.com. May 19, 2006. http://www.webmd.com/a-to-z-guides/chemistry-screen (April 7, 2008).
Maria Basile, Ph.D.
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