Vision tests check many different functions of the eye. Some of the tests measure your ability to see details at near and far distances, check for gaps or defects in your field of vision, and evaluate your ability to see different colors. Others may check how sensitive you are to glare (brightness acuity), how well your eyes work together to provide depth perception, and more. Vision tests are usually done along with exams and tests that check the health of the eye. Here are some common tests that check for blurred or low vision.
Visual acuity (sharpness) tests. These tests help your doctor find out if you have a problem that affects how well you can see. They measure the eye's ability to see details at near and far distances. The tests usually involve reading letters or looking at symbols of different sizes on an eye chart. Usually, each eye is tested by itself. And then both eyes may be tested together, with and without corrective lenses (if you wear them). Several types of visual acuity tests may be used.
Refraction test. This test shows your level of refractive error and finds out the right prescription for glasses or contact lenses. Refractive errors, such as nearsightedness or farsightedness, occur when light rays entering the eye can't focus exactly on the nerve layer (retina) at the back of the eye. This causes blurred vision. Refraction is done as a routine part of an eye exam for people who already wear glasses or contact lenses. But it will also be done if the results of the other visual acuity tests show that your eyesight is below normal and can be corrected by glasses.
Visual field tests. They are used to check for gaps in your side (peripheral) vision. Your complete visual field is the entire area seen when your gaze is fixed in one direction. The complete visual field is seen by both eyes at the same time. It includes the central visual field—which detects the highest degree of detail—and the peripheral visual fields.
Color vision tests. These tests check your ability to distinguish colors. They are used to screen for color blindness in people with suspected retinal or optic nerve disease or who have a family history of color blindness. Color vision tests are also used to screen applicants for jobs in fields where color perception is essential, such as law enforcement, the military, or electronics. Color vision tests only detect a problem—further testing is needed to identify what is causing the problem.
Why It Is Done:
These tests may be done:
As part of a routine eye exam to screen for vision problems. How often you should have routine eye exams changes as you age. Adults and children and teens have different schedules for eye exams.
To monitor an eye problem, such as diabetic retinopathy, or to find out if a treatment is working.
To determine if you need glasses or contact lenses to improve your vision.
After an injury to the eye, to check if your sight was affected.
When you obtain or renew your driver's license or for some types of employment.
To check the near vision of school-age children who have trouble reading, poor school performance, or blurred vision while doing work up close.
This test is done:
To determine the correct prescription for eyeglasses or contact lenses.
To find out if blurred vision is caused by refractive error or eye disease.
Visual field tests
These may be done:
To check for vision loss in any area of your visual field.
To screen for eye diseases, such as macular degeneration and glaucoma, which cause gaps in the visual field.
To look for damage to the nerves of the eye following a stroke, head injury, or other condition that causes reduced blood flow to the brain.
Color vision tests
These tests may be done:
As part of a routine eye exam.
To screen for or diagnose color blindness.
To screen applicants for jobs in which color perception is important, such as truck driving, electronics, or the military.
How To Prepare?
If you wear glasses or contact lenses, bring them with you to the exam since the tests cannot be properly performed without them. If you have a copy of your current eyeglass prescription, bring it with you.
If you have a young child, it is best to practice eye tests at home before you take your child to the appointment. This can help your child cooperate better during the real testing. For more information, see the topic Pediatric Preparation for Medical Tests.
Many medicines may affect the results of vision tests. Be sure to tell your doctor about all the over-the-counter and prescription medicines you take.
Talk to your doctor about any concerns you have regarding the need for vision tests, how they will be done, or what the results will mean
How It Is Done?
Visual acuity tests are used to evaluate eyesight. Several types of visual acuity tests may be used.
The Snellen testchecks your ability to see at distances. It uses a wall chart that has several rows of letters. The letters on the top row are the largest; those on the bottom row are the smallest.
You will stand or sit 20 ft (6 m) from the chart and be asked to cover one eye and then read the smallest row of letters you can see on the chart. If you are unable to cover your eye, an eye patch will be placed over your eye.
Each eye is tested separately. You may be given a different chart or asked to read a row backward to make sure that you did not memorize the sequence of letters from the previous test.
If you wear glasses or contacts, you may be asked to repeat the test on each eye while wearing them.
Let your doctor know if you have trouble reading the letters on one side of the row, or if some letters disappear while you are looking at other letters. You may have a visual field problem, and visual field tests may be needed.
In some people, the dilating eyedrops can cause an allergic reaction.
Vision tests check many different functions of the eye. Your doctor will let you know if your eyesight is normal or if it is better or worse than normal. He or she may also be able to tell you why you have a vision problem.
Why do we need laboratory testing for viral infections?
Various tests are carried out in a laboratory to establish or confirm the diagnosis of a viral skin infection. Although a thorough history and examination of the patient are vital, laboratory tests help the clinician to make a diagnosis.
What is a viral skin swab?
A viral skin swab is a sterile implement lightly rubbed against a visible skin lesion or vesicle. The swab is then sent to the laboratory in viral transport medium for further viral cell culture and virus identification.
What organisms can viral swabs detect?
A viral swab from an external skin lesion or mucosal surface can detect:
Herpes simplex virus (HSV) types 1 and 2 (cause of herpes simplex infections including genital herpes)
Varicella zoster virus (VSV) (cause of chickenpox and shingles)
In addition, a viral swab from oral skin mucosa can detect:
Epstein-Barr virus (EBV) (cause of infectious mononuclosis)
Coxsackie virus A16 (cause of hand, foot and mouth disease)
Viral skin swab
What is a viral cell culture?
Viral cell culture places samples into suitable cell cultures that the virus being tested can infect. When the cells show particular changes the culture is positive. Viral cell culture can identify HSV, VSV, morbillivirus (cause of measles) and other viruses.
What blood tests are needed?
Blood tests for the investigation of viral infections include:
Full blood count: viral infection may raise or reduce the white cell count; atypical lymphocytes may be reported
C-reactive protein (CRP) is elevated but usually <50 in viral infections (CRP is a marker of inflammation anywhere in the body and is not a specific test for viral infections)
Procalcitonin is negative — this is a blood test marker for generalised sepsis due to bacterial infection
Serology: two samples of blood tested 10 days apart to determine immune response to a particular organism.
Polymerase chain reaction (PCR)
PCR involves isolating and amplifying lengths of DNA and then detecting known genetic sequences of microorganisms. It is useful in identifying:
Herpes simplex virus (HSV)
Varicella zoster virus (VZV)
Orf virus (a parapox virus from sheep/goats that causes orf).
Enzyme-linked immunosorbent assay (ELISA)
ELISA can test for specific organisms either by detecting the antigen during a current infection or more commonly, antiviral antibody. The detection of the antibody confirms contact with the virus at some time but it is not necessarily the reason for a current infection.
Most commonly detected viral antibodies are to:
Varicella zoster virus (VSV)
Human immunodeficiency virus (HIV)
Hepatitis B and C virus (in serum), the main causes of viral hepatitis.
What is electron microscopy?
Electron microscopy involves directing a beam of electrons at a sample producing an image. It can provide much higher resolution images than a standard light microscope. It is rarely used in practice. However it can be useful to identify atypical and rare viral infections in the immunosuppressed patients presenting with unusual skin lesions.
What is resistance testing in viruses?
Resistance testing in viruses is done to aid the clinician as to what antiviral medications may or may not be effective for an infection. As viruses have the potential to mutate rapidly, much faster than bacteria or fungi, this presents a therapeutic challenge for the clinician, as treatment with antiviral medications may start to fail. The most noteworthy is HIV antiretroviral (ARV) resistance, which confers a great deal of risk to the patient if the medications stop working.
Sequence analysis is performed most commonly at the genomic level, which looks directly at the HIV genome to see to which antiretroviral drugs that strain of HIV is already resistant and therefore which drugs will not work. Less commonly, this analysis is performed at the phenotypic level, where the patient's HIV virus is subjected to antiretroviral medication in the laboratory to see how well it responds.
What is a viral load test?
A viral load test is a measurement of the amount of HIV virus in a sample of blood. This is usually reported as the number of copies per millilitre (copies/mL). Information about units of measure for blood tests.
Even though less than 1% of HIV is in your blood, viral load is a good marker of how much HIV is in your body. Blood tests are also easier than trying to test lymph nodes or other body tissues.
Viral load tests can also check the amount of HIV in other kinds of sample like genital fluid, semen or spinal fluid.
Types of viral load test
There are three main types of viral load tests:
PCR – polymerase chain reaction (written as PCR RNA). This is the most widely used type of test.
bDNA – branched DNA.
NASBA – nucleic acid sequence based amplification.
These tests work in slightly different ways. PCR is the most widely used viral load test. PCR technology is also used to measure levels of HIV DNA in some circumstances.
How viral load tests work
Viral load tests multiply virus found in a small sample of blood many times so that it can be counted more easily. But this means that the individual results from any one test are not very accurate.
Viral load tests can have a 3-fold margin of error. So, if a viral load result of 30,000, the real result could be anywhere between 10,000 and 90,000 copies/mL.
Each viral load test has a sensitivity cut-off. This is the lowest level of HIV that the test can measure. It is a measure of how sensitive the test is.
The main cut-off limit for viral load tests is less than 50 copies/mL. Many tests now have a lower cut-off of 20 copies/mL.
Special tests can measure down to 5 copies/mL or even to 1 copy/mL. These are only used in research.
Vasectomy reversal is surgery to undo a vasectomy. It reconnects each tube (vas deferens) that carries sperm from a testicle into the semen. After a successful vasectomy reversal, sperm are again present in the semen, and you may be able to get your partner pregnant.
Pregnancy rates after vasectomy reversal will range from about 30 percent to over 90 percent, depending on the type of procedure. Many factors affect whether a reversal is successful in achieving pregnancy, including time since a vasectomy, partner age, surgeon experience and training, and whether or not you had fertility issues before your vasectomy.
Why it's done
Men decide to have a vasectomy reversal for a number of reasons, including loss of a child, a change of heart or remarriage. A small number of men have a vasectomy reversal to treat testicular pain that may be linked to a vasectomy.
Almost all vasectomies can be reversed. However, this doesn't guarantee success in conceiving a child. Vasectomy reversal can be attempted even if several years have passed since the original vasectomy — but the longer it has been, the less likely it is that the reversal will work.
Vasectomy reversal rarely leads to serious complications. Risks include:
Bleeding within the scrotum. This can lead to a collection of blood (hematoma) that causes painful swelling. You can reduce the risk of hematoma by following your doctor's instructions to rest, use scrotal support and apply ice packs after surgery. Ask your doctor if you need to avoid aspirin or other types of blood-thinning medication before and after surgery.
Infection at the surgery site. Although very uncommon, infections are a risk with any surgery and may require treatment with antibiotics.
Chronic pain. Persistent pain after vasectomy reversal is uncommon.
How you prepare
When considering vasectomy reversal, here are a few things to think about:
Vasectomy reversal may be expensive, and your insurance might not cover it. Find out about costs ahead of time.
Vasectomy reversals are generally most successful when they're done by a surgeon who is trained in and uses microsurgical techniques, including those that make use of an surgical microscope.
The procedure is most successful when performed by a surgeon who does the procedure regularly and who has done the procedure many times.
The procedure occasionally requires a more complex type of repair, known as a vasoepididymostomy. Make sure that your surgeon is able to perform this procedure if it's required.
When choosing a doctor, don't be afraid to ask questions about how many vasectomy reversals the doctor has done, the type of techniques used and how often the vasectomy reversals have resulted in pregnancy. Also ask about the risks and potential complications of the procedure.
Food and medications
Make sure you know what steps you need to take before surgery. Your doctor will probably ask you to stop taking certain medications, including blood-thinning medications and pain relievers, such as aspirin or ibuprofen (Advil, Motrin IB, others), because they can increase your risk of bleeding.
Clothing and personal items
Bring tightfitting undergarments, such as an athletic supporter, to wear after surgery. This will support your scrotum and hold bandages in place.
Arrange for someone to drive you home after surgery. Surgery generally takes about two to four hours, or longer. You may need additional time to recover if the procedure is done with general anesthesia. Ask your doctor when you can expect to go home after surgery.
What you can expect
After the procedure
Immediately after surgery, your doctor will cover the incisions with bandages. You'll put on tightfitting undergarments, such as an athletic supporter, and apply ice for 24 to 48 hours to reduce swelling.
You may be sore for several days. If your doctor places bandages over the incisions after your surgery, ask when it's OK to take them off. Any stitches should dissolve in seven to 10 days.
After you return home, take it easy and try to limit activities that might cause the testicles to move around excessively. As the anesthetic wears off, you may have some pain and swelling. For most men, the pain isn't severe and gets better after a few days to a week.
Your doctor may also give you the following instructions:
Wear an athletic supporter for several weeks at all times, except when showering. After that, you'll need to continue to wear one when you exercise.
For the first two days after surgery, avoid anything that might get the surgery site wet, such as bathing or swimming.
Limit any activities that may pull on the testicles or scrotum, such as jogging, sporting activities, biking or heavy lifting, for at least six to eight weeks after surgery.
If you have a desk job, you'll probably be able to return to work a few days after surgery. If you perform physical labor or have a job that requires much walking or driving, talk to your doctor about when it's safe to go back to work.
Don't have sexual intercourse or ejaculate until your doctor says it's OK. Most men need to refrain from ejaculating for two to three weeks after surgery.
Sometime after surgery, your doctor will examine your semen under a microscope to see if the operation was successful.
Your doctor may want to check your semen periodically. Unless you get your partner pregnant, checking your semen for sperm is the only way to tell if your vasectomy reversal was a success.
When a vasectomy reversal is successful, sperm may appear in the semen within a few weeks, but it can sometimes take a year or more. The likelihood of achieving pregnancy depends on various factors, including the number and quality of sperm present and the female partner's age.
Why Get Tested?
To help diagnose or rule out a neuroblastoma or other neuroendocrine tumor; to monitor the effectiveness of treatment.
When To Get Tested?
When your child has a lump in the abdomen (abdominal mass), bruising around the eyes, trouble walking, or bone pain; when you have symptoms of increased catecholamine release, such as persistent or periodic high blood pressure, severe headaches, rapid heart rate, and sweating; when you have been treated for a neuroendocrine tumor, such as neuroblastoma.
A 24-hour urine sample is preferred; sometimes a random urine sample is acceptable.
Test Preparation Needed?
This test is affected by caffeine, certain foods, certain drugs, and stresses. Inform your healthcare practitioner of any medications you are taking and follow any instructions you are given for things to avoid before sample collection.
How is it used?
The vanillylmandelic acid (VMA) test is primarily used to detect and rule out tumors called neuroblastomas in children with an abdominal mass or other symptoms suggestive of the disease. It may be ordered along with an homovanillic acid (HVA) test to help diagnose a neuroblastoma, to monitor the effectiveness of treatment, and to monitor for neuroblastoma recurrence.
A urine creatinine test is typically also ordered, especially with random urine testing, and tests results are frequently reported as VMA-to-creatinine and HVA-to-creatinine ratios. Since creatinine is a substance that is passed into the urine at a relatively constant rate, it serves as a comparison to the amounts of VMA and HVA.
A VMA test was once frequently ordered to detect pheochromocytomas, but the preferred tests are now plasma free metanephrines, urine metanephrines, and urine or plasma catecholamine tests. The VMA test may still be ordered along with one or more of these tests to help detect and rule out a pheochromocytoma.
When is it ordered?
VMA testing is ordered when a healthcare practitioner either suspects that a child has a neuroblastoma or wants to rule out the possibility. A healthcare practitioner may order it, along with an homovanillic acid (HVA) test, when a child has an abdominal mass or other symptoms suggestive of a neuroblastoma. These signs and symptoms can vary depending upon their location and may vary over time:
Most neuroblastomas are located in the belly, so the most common symptom is an unusual non-tender lump or mass in that area. Others include abdominal pain and lack of appetite.
Sometimes a tumor in the neck can cause swelling and affect swallowing or breathing.
If the tumor presses on nerves by the spine, it may affect the child's ability to walk.
If it has spread to the bones (metastasized), it may cause bone pain, and if to the marrow, it may affect blood cell production, causing fatigue, pallor, and bleeding.
What does the test result mean?
Since the VMA test is sensitive to many outside influences, and neuroblastomas and pheochromocytomas are rare, the false-positive rate for this test is high – greater than 15%. When a person has large amounts of VMA in his or her urine, it is not diagnostic of a condition; it is an indication that further investigation is necessary.
Intense emotional and physical stresses can cause moderate to large temporary increases in VMA levels. The healthcare practitioner must evaluate the child or adult as a whole – his or her physical condition, emotional state, prescribed medications, and any over-the-counter products that the person is taking. When interfering substances and/or conditions are found and resolved, the healthcare practitioner will frequently re-test the person to determine whether the VMA is still elevated. The healthcare practitioner may also order other catecholamine and metanephrine testing, genetic mutation testing, and imaging tests such as an MRI to help find the tumor(s).
Is there anything else I should know?
The amount of VMA produced does not necessarily correspond to the size of the tumor. This is a physical characteristic of the tumor tissue. The total amount of VMA produced will tend to increase, however, as the tumor increases in size.
A healthcare practitioner will sometimes evaluate a VMA-to-HVA ratio with a neuroblastoma. A low ratio is associated with a poorer prognosis.
A variety of medications can interfere with VMA testing, but those being tested should always consult with their healthcare practitioner before discontinuing any prescribed medications. Some of the drugs that can interfere include appetite suppressants, caffeine, clonidine, disulfiram, histamine, imipramine, insulin, epinephrine, levodopa, lithium, morphine, MAO inhibitors, nitroglycerin, and rauwolfia alkaloids. The effects of these drugs on VMA results will be different from person to person and are often not predictable.
Why do i have to collect my urine for 24 hours?
The reason that you need to collect your urine over a 24-hour period is because the amount of VMA released in the urine changes during the course of a day. By collecting all urine for 24 hours, the amount of VMA measured can be averaged over the entire day and will give a better indication of whether levels are increased or not