Platform
Company
Immunology & Autoimmune
Review status
Currently under review
Pending specialist review and validation.
The Alternative Pathway test measures how well one arm of your complement system works. The complement system is a group of blood proteins that helps your immune system fight germs and clear damaged cells. This test evaluates the function of proteins that make up the alternative pathway, which can be activated without antibodies and provides rapid, early defense against infection.
Results are typically reported as a percentage that reflects activity compared with a normal reference sample. The test is performed on serum and reflects the overall ability of the alternative pathway to activate and carry out its effects, rather than measuring a single protein.
Doctors use this test to check for inherited or acquired problems with the alternative complement pathway. Low activity can be seen in certain genetic deficiencies, and it can also occur when the pathway is being heavily used or regulated during illnesses. It is often ordered when you have recurrent or unusual infections, especially with certain bacteria, or when there are signs of immune system overactivity or kidney inflammation.
The test also helps monitor conditions where complement is targeted by treatments, such as medications that block specific complement proteins. It is commonly interpreted together with other complement tests, like total complement activity through the classical pathway and individual components such as C3 and C4, to get a full picture of your complement function.
If your alternative pathway activity is lower than expected, it may suggest a deficiency in one of the pathway proteins or that the pathway is being consumed during an active illness. Your clinician may repeat the test to confirm the result and may order additional tests for specific complement components to pinpoint the cause.
Higher than expected activity can occur in some inflammatory states or may reflect normal variation. Your results will be interpreted in the context of your symptoms, medical history, and other lab findings. If results do not match how you are feeling, your clinician may recommend follow-up testing, including other complement pathway assays or genetic testing, to clarify the picture.
Reference intervals vary by laboratory, analyzer, methodology, population, and units. The ranges shown here are for education only. Always interpret your results against the reference interval printed on your own lab report.
Complement activity is sensitive to delays in processing, improper storage, and repeated freeze–thaw cycles. Serum should be collected, separated, and frozen promptly; poor handling can falsely lower activity.
Active infections and inflammatory states can change complement protein levels and turnover. This may increase or decrease measured activity depending on timing and disease activity.
Therapies such as C5 inhibitors (for example, eculizumab or ravulizumab), C3 inhibitors (for example, pegcetacoplan), or factor D inhibitors can markedly reduce alternative pathway activity.
Complement proteins can rise during pregnancy, which may influence functional results. Your clinician will interpret values with pregnancy status in mind.
Recent transfusion or infusion of plasma or complement-containing products can temporarily alter measured activity and may mask an underlying deficiency.
Newborns and young infants can have lower complement activity compared with adults. Age is considered when interpreting results.
References