Diagnosing

Pilonidal Abscesses are classified by the medical community as a disease of the colon and rectum even though they affect neither.

A Pilonidal is, at the most basic level, a boil that has gone very, very bad.  Pilonidals are not technically cysts since they do not form a sac. They are an abscess that is created from an infected follicle.

Self Diagnosis Basics: 

Pilonidal Abscesses usually form just to the side of the top crease in the buttocks. The sinus opening(s) (if there is one) are typically along the center (midline) of the crease. The lump can be as small as a pea or larger than a golf ball.  Usually the lump will hurt when you press on it - if it's infected it will hurt A LOT. If you have a sinus, it will typically drain a nasty smelling discharge that can be brownish, whitish, clear or streaked with blood. Some people don't get a "lump" if they have a draining sinus - the lump forms as infected fluid builds up inside the cyst.  Some abscesses can be very deep in the tissue and there will be no visible sign at all.

Many people note lower back pain as problems with their abscess escalate.  There is no known tie, but theories from the message board have suggested that the pressure from the cyst is crowding nerve endings heading to the spinal cord, most likely the sciatic nerve.  It also has been suggested that most Pilonidal sufferers tend to "sit funny" to try and get comfortable.  This throws the back out of normal alignment.  Most people who have lower back pain have found that it disappears after the abscess is treated or removed.

Pilonidals tend to take one of several courses:

Acute: excruciating pain in the tailbone area, with an obvious inflamed lump at the top of the buttock cleft. This one is a pretty easy call and any doctor who misdiagnoses this should lose their license to practice. When a Pilonidal Cyst is infected (inflamed, acute, abscessing, flaring up) it will swell and make it almost impossible to sit, stand or lie down since any movement seems to make the pain worse. A lot of people first hear the word Pilonidal when they are carried into an ER screaming in pain. 

Messy but painless: for those who have sinus holes, their experience with the disease may never reach the acute stage since the holes allow the abscess to drain rather than swell. Some people have minimal drainage with no odor, other people have rivers of drainage with an unmistakable stench. Frequently the drainage will cause itching, which is your skin responding to the fluid being excreted from the abscess. 

Stealth Mode: some people have lumps that they can feel, but cause very little pain or sinus holes that only drain a few times a year. If this is you, consider yourself VERY LUCKY.  Be aware that a sleeping giant can awaken and turn into either of the above unpleasant scenarios at any time.

Tests: 

There is not a lot of testing that is applicable to Pilonidal Abscesses.  

X-Rays will not see Pilonidal Cysts.  What they may be useful for, however, is detecting Spina Bifida Occulta, which may be the cause of congenital cysts (Sacral Dimples.)

MRI uses radio frequency radiation in the presence of a controlled magnetic field to produce high quality images.  MRI scans will not usually see a Pilonidal Cyst unless the scan is set at a very high resolution. The scans can detect soft tissue swelling that indicates probable Pilonidal activity. There are two types of MRI - with contrast or without.  With contrast involves using a dye and provides a much better view of masses such a cysts.  MRI assisted surgery is being used in some Colo-Rectal applications. 

MRI pictures of a Pilonidal Cyst:

What else it could be:

The list of usual suspects includes - 

1) Coccydynia - rare condition of pain in the Coccyx.  http://www.coccyx.org/.  

2) If the infected area is closer to the anus, it could also be an Anal Fistula (a tunnel that goes between the colon and the outer portion of the anus) or a Peri-Anal Abscess.
http://www.fascrs.org/brochures/anal-abscess.html

3) It might be a condition in the same family as Pilonidal called Hidradenitis suppurativa, which sometimes forms several "boil" like pustules in areas of the buttocks.
www.hs-usa.org

4) There is also another very rare condition called Chordoma (a rare, slow-growing, but locally aggressive malignant tumor) that was discovered during treatment for a presumed Pilonidal Cyst. 

Correct diagnosis by a "butt doctor" is a good idea if your abscess is located lower in the buttock crease where it is possible to be an Anal Abscess or Fistula.