Tests include complete blood count with reticulocyte count and peripheral smear to detect RBC clumping; hemolysis markers (bilirubin, LDH, low haptoglobin); direct antiglobulin (Coombs) test often positive for complement (C3); cold agglutinin titer and thermal‑amplitude testing; serum protein electrophoresis/immunofixation for monoclonal IgM; and evaluation for infections (Mycoplasma, EBV) or other underlying causes.

There is no specific blood test for the common cold. Colds are diagnosed clinically; if testing is needed, respiratory viral PCR or antigen tests from nasal/throat swabs identify viruses. Blood tests such as a complete blood count or inflammatory markers (CRP) can assess severity or complications. Serology (IgM/IgG) may detect specific viral infections but isn’t routinely used for typical colds.

Cold agglutinins are not directly detected on a routine complete blood count. A CBC may show indirect clues — e.g., spurious RBC clumping, unexpectedly high MCHC, low RBC count, anemia or hemolysis features — but confirmation requires specific testing such as a cold agglutinin titer and a direct antiglobulin (Coombs) test. Clinicians correlate symptoms and lab clues before ordering targeted tests.

The agglutinin test detects antibodies that cause red blood cells to clump (agglutinate). It’s used to diagnose autoimmune hemolytic anemia and cold agglutinin disease, and to identify antibody responses linked to infections such as Mycoplasma pneumoniae. Results help explain unexplained hemolysis, guide transfusion compatibility, and inform treatment choices like immunosuppression or infection-directed therapy.

Cold agglutinins are typically IgM autoantibodies that bind red blood cells at cold temperatures, fix complement, and cause complement-mediated hemolysis and agglutination. These IgM antibodies often target common red‑cell antigens (eg, the I antigen). Rarely, cold-reactive IgG antibodies occur (for example, the Donath–Landsteiner antibody in paroxysmal cold hemoglobinuria), but classic cold agglutinin disease is IgM-mediated.

A cold agglutinin test requires a blood draw and lab processing; the titration itself typically takes a few hours. Many laboratories can report results the same day, but if samples are sent to a reference lab or need confirmatory testing, turnaround may be 24 to 72 hours. Urgent inpatient tests are often expedited, so timing varies with workload and assay complexity.