| Subject | Contents |
| Definition | A test performed to detects and measure the percentage of abnormal hemoglobin derivatives that result from drugs such as nitroglycerin or poisoning by carbon monoxide. |
| Alternative Names | Methemoglobin; Carboxyhemoglobin; Sulfhemoglobin; Carbon monoxide poisoning |
| How the test is performed | Adult or child: Blood is drawn from a vein ( venipuncture ), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill with blood). A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding . Infant or young child: The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding. The samples are then sent to the lab where the percentage of total hemoglobin verses derivatives is evaluated. |
| How to prepare for the test | If your child is to have this test performed it may be helpful to explain how the test will feel, and even practice or demonstrate on a doll. The more familiar your child is with what will happen to them, and the purpose for the procedure, the less anxiety they will feel. |
| How the test will feel | When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing. |
| Why the test is performed | Hemoglobin is the substance located within red blood cells that transports oxygen and carbon dioxide between the lungs and body tissues. If certain chemicals or drugs are introduced into the blood stream, they can alter the hemoglobin so it can no longer function as a transport medium between the lungs and tissue. Measurement of these abnormal hemoglobin derivatives can be helpful in diagnosing and monitoring exposure to these agents.
Carboxyhemoglobin is hemoglobin bound to carbon monoxide instead of oxygen or carbon dioxide. Carbon monoxide has a much higher affinity for hemoglobin than does oxygen (210 times more), and high percentages of carboxyhemoglobin in the blood impairs the normal transport of oxygen by the blood. Sulfhemoglobin results from the union of hemoglobin with medications such as phenacetin or sulfonamides. The resultant form of hemoglobin is unable to transport oxygen, and is untreatable. The only treatment is to wait until the affected red blood cells are destroyed as part of their normal life cycle.
Methemoglobin occurs when the iron which is part of the hemoglobin is altered such that it does not carry oxygen well. Certain compounds introduced into the blood stream can cause this oxidation: sulfonamides chlorates nitrates nitrites aniline phenacetin
There is also a congenital form of this disorder caused by a deficiency of NADH cytochrome b5 reductase , and another form caused by hemoglobin M disease.
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| Normal Values | The following values represent the percentage of hemoglobin derivatives based on total hemoglobin : carboxyhemoglobin: 3% (but may be as high as 15% in smokers) methemoglobin: less than 3% sulfhemoglobin: undetectable |
| What abnormal results mean | As abnormally high levels of hemoglobin derivatives rise, there are increasingly detrimental effects. Since these forms of hemoglobin do not permit the transfer of oxygen, the effects on the body are related to lack of oxygen to the tissues, which can result in tissue death. The following values except sulfhemoglobin, represent the percentage of hemoglobin derivatives based on total hemoglobin: Carboxyhemoglobin: 20% symptoms of carbon monoxide poisoning begin to appear 30% severe carbon monoxide poisoning present 50% to 80% results in fatal carbon monoxide poisoning Methemoglobin: 10% to 25% result in cyanosis 35% to 40% result in shortness of breath and headache over 60% results in lethargy and stupor over 70% results in death Sulfhemoglobin values of 10 gm/dl result in cyanosis, but do not produce toxic effects Note: lower levels are hemoglobin derivatives produce the same effects in children. gm/dl = grams per deciliter |
| What the risks are | Risks associated with venipuncture are slight: excessive bleedingfainting or feeling light-headed hematoma (blood accumulating under the skin) infection (a slight risk any time the skin is broken) multiple punctures to locate veins |
| Special considerations | Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others. |
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