The reason is simple: There are more than 20 sexually transmitted diseases out there, and it wouldn’t make sense to do 20 different tests when you go the doctor. Instead, you get certain STD tests based on whether or not you:
are just getting a routine check-up and don’t have any symptoms
have genital bumps or sores
practice certain sexual behaviors
find out your partner has an STD
just want an HIV test
Here are some examples of each reason you might get tested for STDs:
Reason 1: It’s a Routine Check-Up, You Have No Symptoms
Example: You’re a guy getting a routine check-up. Your urine can be checked for leukocytes (white blood cells) which could mean an infection with gonorrhea or chlamydia. To be sure, you get a urethral swab to make the diagnosis.
Or -- if the test is available to your doctor -- your urine itself gets tested for gonorrhea and chlamydia with new tests called nucleic acid amplification tests. In that case, you avoid the urethral swab altogether.
Example: You’re a girl getting a routine check-up. During the pelvic exam, you get tests for gonorrhea and chlamydia. A PAP test is done to check your cervix for signs of infection with human papilloma virus (HPV), the cause of genital warts.
And just like with guys, your urine can be tested directly for gonorrhea and chlamydia, so you can skip the pelvic exam. However, if you are due for your annual PAP test, you would still need the pelvic exam. That’s still the only way a PAP test can be done.
So why do guys and girls need routine STD tests if they’ve got no symptoms? Because gonorrhea and chlamydia very often cause silent infections -- meaning you’ve got ‘em and can spread ‘em and not even know it.
For guys and girls:
Your genital skin can be examined for any bumps caused by genital warts or molluscum contagiosum -- another virally transmitted STD.
You can get a routine HIV test.
You can get a blood test for syphilis and hepatitis C if you’ve had a lot of sexual partners.
Reason 2: You Have Symptoms (discharge from penis or vagina, pain or burning with urination)
Example: You’re a girl with vaginal discharge. You need a pelvic exam to get tested for gonorrhea, chlamydia and trichomonas, and for other causes of discharge like yeast infections and bacterial vaginosis (BV). The pelvic exam also checks for pelvic inflammatory disease (PID) -- an infection of your internal reproductive organs (uterus, Fallopian tubes, ovaries).