What Is Enzyme Immunoassay (EIA)?

What to Expect When Undergoing This Test

An enzyme immunoassay (EIA) or an enzyme-linked immunosorbent assay (ELISA) is a blood or urine analysis that can help in the diagnosis of many infections and inflammatory conditions. This is a simple test that does not cause side effects. Your EIA can help in diagnosing the cause of your symptoms and is used to guide your therapy.

Often, you might not specifically know that you are going for an EIA test unless you carefully check your order form. EIA is a laboratory technique, but it does not alter the way your blood is collected.

This test works by producing a color change in a solution when your sample is examined in the lab. The color change occurs as the result of a chemical reaction described as an enzyme-antibody reaction.

Purpose of Test

An EIA test is used for children and adults. You may need to have an EIA test if you develop symptoms or have had an exposure to certain infections. This test is also used to help in the diagnosis of allergies and some autoimmune conditions.

An EIA test can also be used for identifying certain medications and for drug screening.

Some of the infections that can be detected by EIA include:

Other substances that can be detected by an EIA test include:

  • Cancer markers: Cancer markers are proteins or hormones made by some types of cancer and they may be detected before cancer is large enough to be seen on imaging tests
  • Hormone levels: Many steroid hormones and reproductive hormones can be detected with this test.
  • Inflammatory markers : Inflammatory and autoimmune diseases may produce antibodies or other proteins that can be detected by EIA.
  • Certain medications: Opioids, pain medications, and sedatives are among the long list of medications that can be identified using EIA.

Allergy Testing

An EIA test can be used to help identify whether you are allergic to a particular substance. An allergy is characterized by an antibody (immune protein) that reacts to an otherwise harmless substance described as an allergen. An allergen is a substance that induces an allergic reaction.

Drug Screening

Another use of EIA is for drug screening. A blood or urine sample can be used to detect the presence of certain drugs, such as cocaine or hallucinogens. This is typically used in a setting such as an employment associated drug screen.

Risks and Contraindications

An EIA is a simple test without risks. If you are healthy enough to tolerate having your blood drawn, then you should not have any contraindications to having this test.

If you have bleeding problems, hypotension (low blood pressure), or anemia (low blood cell count and/or red blood cell function), your doctor may give you special precautions when you have this test.

Before the Test

Typically there is no preparation necessary before having an EIA. If your doctor wants you to stop taking medications or to discontinue exposure to any substances, you will be given specific instructions before your test.

Timing

You should be prepared to spend about an hour at the facility where you will have your blood or urine collected. It is a good idea to ask about the anticipated duration of your wait time before you go for your test, especially if you have other things planned for your day.

The blood collection procedure itself will typically only take about five minutes, but the time spent signing in, filling out paperwork, and waiting for your turn can take longer.

Location

You can go to your doctor’s office, a laboratory in a medical building, or a hospital to have your blood or urine collected for your EIA. Your doctor and your health insurance provider may provide you with a list of approved locations where you can go to get this test.

What to Wear

For an EIA, your blood will usually be collected from a vein in your antecubital area (the inside of your arm at the level of your elbow) or from a vein in your hand.

When you are going to have this test, you should wear clothes with short sleeves or with sleeves that are easy to roll up above your antecubital area. Often, people who are going to work after having their blood drawn like to wear long sleeves to cover the bandage.

If you will have a urine test, it is a good idea to wear clothes that are easy for you to pull up or down so that you can collect your urine while holding the specimen container.

Food and Drink

Unless your doctor gives you specific instructions to avoid a certain food or drink, you can eat and drink as usual before your test without any restrictions.

Cost and Health Insurance 

Depending on your health insurance plan, the cost of this test may be completely or partially covered. You may have to pay co-pay even if your health insurance covers most of the cost of the test. It is best to check with your health insurer in advance and to check the cost with the laboratory where you will have the test done.

An EIA blood test costs between $50 and $200. Keep in mind that this cost may be higher if you are having tests for multiple allergens. An EIA urine test costs between $20 and $40. This may vary depending on how many substances are being tested.

The cost an employment drug screening test is not likely to be covered by your health insurer, and either you or your employer will have to pay it.

What to Bring

Be sure to bring your insurance card, an identification card, and some method of payment with you when you go for this test. Before being permitted to have the test, you may be asked to pay your copay or to provide the full payment if you will be paying for the test yourself.

During the Test

When you go for your EIA test, you will meet with staff who will help you get signed in and fill out your forms. You will also meet with a nurse or phlebotomist who will collect your blood or help you with the urine test.

Pre-Test

Before the test, you will be asked to sign in, show the order for the test, and present your identification and insurance information. You may also be asked to wait for your turn if they are not ready for you right away.

Throughout the Test

A nurse or phlebotomist will call you in to have your blood drawn. You will be asked to sit. Usually, they will also ask you which hand you write with or which arm you prefer to have your blood drawn from. They generally try to avoid drawing blood from the arm that you write with.

You will have a tourniquet or a band wrapped around your upper arm. Then your nurse or phlebotomist will feel for the vein and cleanse the area around your vein.

You will then have a small needle inserted into the vein, and typically a tube will be placed on the other side of the needle to collect the blood. You will have the needle inserted into your arm for less than a minute. When the needle is withdrawn, a small cotton ball or gauze will be placed on the puncture site with some pressure to stop the bleeding. You may be asked to put some pressure on it yourself for a few seconds. 

A Urine Specimen

If you are having a urine test, you may be instructed on how to collect your urine specimen. You will be shown to a private bathroom, given a container to collect your urine, and instructed to clean yourself prior to the collection to prevent contamination. 

Post-Test

Once the bleeding has stopped (usually this takes less than a minute), a bandage will be placed over the puncture site and you will be able to leave. 

If you experience any dizziness or lightheadedness or if you are continuing to bleed from the puncture site after your test, be sure to tell your nurse or phlebotomist.

You can drive yourself home after the test, and you do not need to be accompanied by anyone. After the test, you should leave the bandage on the puncture site for about 24 hours to keep it clean.

After the Test

For the first day after your test, it is a good idea to avoid lifting heavy objects with the arm that was used to draw blood.

Most people have no problems after an EIA test. Some people notice soreness or minor bruising around the puncture site, but this should not last longer than a few days.

Managing Side Effects 

If you are in frail health or if you have anemia, you may experience some side effects after having your blood drawn. Symptoms can include lightheadedness, dizziness, and fatigue.

If you have a bleeding condition, your puncture site may continue to bleed.

If you have severe or persistent pain, bruising, bleeding, or lightheadedness, be sure to talk with your doctor about it.

Interpreting Results

EIA test results will generally take a few hours to a couple of days. Your doctor should be able to give you a time frame in which to expect the results, and whether you will be notified, should call to check, or can receive them electronically.

How EIA Works 

Your EIA test interpretation is based on a color change. This test can be read as positive or negative, and it can also produce a quantitative result, which compares the reaction to the standard or normal range.

Because this test is used for so many different indications, each indication has its own normal range and standard result, which will be included with your test report.

An EIA uses a sample of your blood or urine. The sample is exposed to a protein that is known to bind to a very specific substance, such as an antibody. Because EIA is used for a wide variety of diagnoses, the protein used for your test depends on which condition you are being tested for. For example, if you are having an EIA to diagnose hepatitis B, this test cannot be read as positive or negative for a different condition, such as HIV.

Once your sample is exposed to a selected protein, a substance in your blood sample may bind to the protein. After this initial step, residue is washed away and an enzyme is introduced to the solution. The enzyme is selected because it binds to the protein that was initially placed with your sample. If a binding reaction takes place, then the enzyme induces a color change.

If the color of the solution changes, this is considered an indication that the substance that is being tested for is present, and it is considered a “positive” test. If no color change occurs, this is considered a negative test.

Sensitivity and Specificity

EIA tests are very sensitive and very specific. The antibodies that are used in an EIA test only bind to the substance being tested, and not to anything else, making the result specific. The color change can occur in response to a very small quantity of the substance being tested, making these tests very sensitive.

That said, all tests can produce false positives (they inaccurately say that you have a diagnosis even when you don’t) and false negatives (they inaccurately say that you don’t have the diagnosis even when you do have it). The rate of false positivity or false negativity is different for each condition that is tested for by EIA.

Follow-Up

You may need a follow-up EIA in some situations. If you are being tested for an allergy, you may need a repeat test after a time interval to assess whether you have had any change in your allergic reaction.

When your EIA test was used for detecting an infection, you may not need to have it repeated if your symptoms improve with treatment. However, some infections, like HIV, need to be monitored, and you may need to have the test repeated at regular intervals.

If you had your EIA for a work screening drug test, then repeat testing is based on your employer’s policy.

A Word From Verywell

There is a strong chance that you may need to have an EIA test at some point. The test is very safe with rare adverse effects.

The results need to be interpreted in light of standard measures and also with attention to your overall health and symptoms. This test can be helpful for many indications—and results can provide information about your health that other tests do not typically provide.

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Article Sources

  1. Hu KQ, Cui W, Rouster SD, Sherman KE. Hepatitis C virus antigens enzyme immunoassay for one-step diagnosis of hepatitis C virus coinfection in human immunodeficiency virus infected individuals. World J Hepatol. 2019 May 27;11(5):442-449. doi: 10.4254/wjh.v11.i5.442

  2. Orme ME, Andalucia C, Sjölander S, Bossuyt X. A comparison of a fluorescence enzyme immunoassay versus indirect immunofluorescence for initial screening of connective tissue diseases: Systematic literature review and meta-analysis of diagnostic test accuracy studies.Best Pract Res Clin Rheumatol. 2018 Aug;32(4):521-534. doi: 10.1016/j.berh.2019.03.005. Epub 2019 Apr 15.

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