C3 and C4 together
The combination of indicators helps determine which pathway of complement activation is involved. Reductions in both C3 and C4 indicate the classical pathway and are typical of systemic lupus erythematosus and cryoglobulinemia; an isolated decrease in C3 with normal C4 is characteristic of the alternative pathway and post-streptococcal glomerulonephritis. A separate important indication for C4 is hereditary angioedema: in this disease, the level of C4 is persistently reduced even outside of attacks, which makes it a convenient screening test in patients with recurrent swelling of the face, larynx and abdomen without urticaria, not responding to antihistamines.
Why preparation is so important
The laboratory result is a number, but the correct number can only be obtained if the rules are followed. Eating changes the levels of glucose, lipids and a number of enzymes significantly; physical activity the day before increases CK and AST; smoking affects hormones; time of day is critical for cortisol and testosterone.
Therefore, a “bad analysis” often turns out to be a consequence of impaired preparation, and not an illness, and leads to unnecessary examinations and prescriptions.
General rules for donating blood
- on an empty stomach 8–12 hours, you can drink water
- Eliminate alcohol and intense exercise per day
- no smoking in an hour
- donate blood before physiotherapy, x-rays and massage
- Take medications in advance by agreeing with your doctor
- for hormones in women, take into account the day of the cycle
How to read the result
Reference values depend on the method and equipment of the laboratory, so it is correct to compare results only within the same laboratory. A deviation of a few percent from the normal limit in itself rarely matters - the overall picture and dynamics are important. It is up to the physician to interpret the analysis: an isolated indicator without a clinical context is often misleading.