80% of people with P antigen are P1 phenotype (have P and P1 antigens) and 20% of people with P antigen are P2 (P antigen only - no P1). The P2 phenotype merely signifies absence of P1 antigen in people with P antigen (globoside). Individuals who are Pk consistantly have an alloanti-P that is an IgM antibody. The absence of the P antigen is very rare and is designated as p.
Auto anti-P is an IgG antibody seen in P1 or P2 persons and causes clinically significant Paroxysmal Cold Hemoglobinuria (PCH). The Donath-Landsteiner Test helps diagnose PCH. It is designed to detect biphasic anti-P. A biphasic antibody reacts with the antigen on the red blood cell at colder temperature, binding complement . When antigen-antibody-complement complex warms up, the red blood cells are hemolyzed. Donath-Lansteiner test is performed by incubating patient's serum with his own cells first at 4oC, then at 37oC, and then looking for hemolysis.
Paroxysmal Cold Hemoglobinuria.
The classic symptom of PCH is a sudden onset of hemoglobinuria following exposure to cold, even for a few minutes. Symptoms may occur minutes to hours following exposure to cold. Hemoglobinuria is not always present because, in some persons with PCH, the autoantibody level is not high enough to cause intravascular hemolysis. In other individuals with PCH, hemolysis may occur spontaneously without exposure to cold. Anemia may range from mild to severe. A recent respiratory or other viral infection is common.
Symptoms of anemia
Symptoms of hemolysis
Dark urine caused by hemoglobinuria
Pain or cramps in the back, legs, or abdomen
Nausea, vomiting, diarrhea
Tingling of the hands and feet
A fever as high as 40°C is common.
Signs of anemia include pallor and tachycardia.
Signs of congestive heart failure or shock are rare but may be present when anemia is severe.
Signs of hemolysis include jaundice and splenomegaly.
Vasomotor signs include cyanosis, Raynaud phenomenon, and gangrene.
The 3 main causes of PCH are congenital or tertiary syphilis, a postinfectious and/or postvaccination etiology, and idiopathic and/or autoimmune causes. PCH associated with chronic syphilis was previously common but now is observed rarely. Acute transient postinfectious PCH is more common. Chronic idiopathic and/or autoimmune PCH is rare.
PCH associated with chronic congenital or tertiary syphilis
Acute transient postinfectious and/or postvaccination PCH
Viral infections, including measles, mumps, influenza A, adenovirus, varicella, cytomegalovirus, and Epstein-Barr virus
Bacterial infections, including Haemophilus influenzae and Mycoplasma pneumoniae
Vaccines, including measles vaccine
Chronic idiopathic and/or autoimmune PCH
This would not be the sort of test done in a regular laborotory. It would have to be done in a reference lab. I work in a hospital lab and we do not do this test.
We run across quite a few people who have Anti-P antibodies when screening for crossmatches and this is not a clinically significant antibody in crossmatching. We just give crossmatch compatible blood.