A man's inability to achieve or maintain an erection (erectile dysfunction or impotence) is a common problem. Most erection problems are due to a combination of blood vessel, nerve, and psychological factors. It is necessary to determine the specific cause to correct the problem.

Physical conditions that may cause erectile dysfunction include:

• Problems with the nerves of the penis. Diseases such as multiple sclerosis or diabetes may cause nerve damage that affects a man's ability to have an erection. Also, surgery, injury, or radiation treatment to the pelvic area may cause damage to the nerves of the penis.
• Problems with the blood vessels that supply blood to the penis. Conditions such as hardening of the arteries (atherosclerosis) may make it difficult for a man to achieve an erection. Activities that constrict blood flow to the penis (such as long distance bicycle riding on a hard, narrow saddle) may increase a man's risk for developing an erection problem. 
• Low levels of hormones, such as testosterone or thyroid hormones.
• Medication side effects (for example, medications taken for high blood pressure or depression). The use of tobacco, alcohol, or illegal drugs can lead to erection problems.

Psychological causes of erectile dysfunction may include:

• Anxiety about sexual performance.
• Relationship problems.
• Stress.
• Depression or grief.

Initial testing for erection problems includes a thorough history (including sexual history) and physical exam. A urine test may be done. Several blood tests may be done, including a complete blood count (CBC), testosterone, luteinizing hormone, prolactin, thyroid hormones, blood glucose, and cholesterol. For more information, see the medical tests Urine Test, Complete Blood Count (CBC), Testosterone, Prolactin, Thyroid Hormone Tests, Blood Glucose, and Cholesterol and Triglycerides Tests.

Additional testing for erection problems may include a nocturnal penile tumescence (NPT) test or an intracavernosal injection test.

Nocturnal penile tumescence (NPT) test

The NPT test is also called the stamp test or the rigidity test.

Most men have 3 to 5 full erections during deep (rapid eye movement or REM) sleep. Men who are unable to have an erection because of a psychological problem still have erections during deep sleep. Occasionally, some sleep disorders or serious depression can prevent these nighttime (nocturnal) erections.

The NPT test determines whether a man is having normal erections during sleep. This test can be done at home or in a special sleep lab. One of three methods may be used (from least to most complicated):

• The man places a ring of perforated paper stamps around his penis before going to sleep. When he wakes up, he checks to see if the perforations between the stamps are broken, indicating that he had an erection during the night. This test is not very reliable because the stamps can loosen and fall off.
• The man uses a simple ringlike device (called a snap gauge) instead of stamps. It works the same way but is more reliable than stamps.
• The man uses an electronic monitoring device. This method is more expensive than the other two, but it is more accurate and provides more detailed information about the number, duration, and hardness of erections that occur.

Testing is usually repeated for at least two nights. If good erections occur during sleep, the major cause of the erection problems probably is not physical.

Intracavernosal injection test

During this test, the doctor injects a medication (usually prostaglandin E₁) into the base of the penis to produce an erection. This is called an intracavernous injection. A similar medication may also be placed directly into the tube through which urine leaves the penis (urethra). This is called an intraurethral medication.

The fullness, stiffness, and duration of the erection are then measured.

Tests for Erection Problems
Results

For information on the results of initial tests for erection problems, see the medical tests Urine Test, Complete Blood Count (CBC), Thyroid Hormone Tests, Blood Glucose Tests.

Nocturnal penile tumescence (NPT) test

An erection has probably occurred if:

• The stamp perforations have broken.
• One or more films on the snap gauge have broken.
• The electronic device has registered at least one increase in the size of the penis.

If you have had an erection, your erection problems may have a psychological rather than physical cause. The test is more reliable if repeat test results show similar results.

An erection has probably not occurred if:

• The stamp perforations have not broken.
• None of the films on the snap gauge have broken.
• The electronic device has not registered an increase in the size of the penis.

If you have not had an erection, your erection problems may have a physical cause. The test is more reliable if repeat test results show similar results.

Intracavernosal injection test

An erection produced during the intracavernosal injection test is usually measured on a scale of 0 to 4, with 4 representing a full erection.

• If the test produces an erection with a value of 3 or 4, it is likely that your erection problems have a psychological rather than physical cause.
• If the test produces an erection with a value of 0 to 2, your erection problems may have a physical cause.