Solving the Puzzle: Diagnosis and Lab Reports

Diagnosing a rare complement deficiency requires a specific series of blood tests that act like a funnel: they start with broad “activity” screens and narrow down to the specific protein and gene at fault ^[1]. Because these proteins are highly sensitive to handling, the way your blood is managed in the lab is just as important as the test itself ^[2][3].

The First Step: Functional Activity Screens

To see if your complement system pathways are working, doctors look at two main functional tests. Think of these as measuring the “output” of a factory line rather than counting the individual workers.

• CH50 (Classical Pathway): In complete C1r/C1s deficiency, the CH50 is typically absent or immeasurably low (zero or near zero) ^[4]. This tells the doctor the classical pathway is broken.
• AH50 or AP50 (Alternative Pathway): This measures a different branch of the complement system. In your condition, the AH50 should be normal, because the alternative pathway does not require C1r or C1s to function ^[4][1].

Important: An absent CH50 with a normal AH50 strongly suggests an early classical-pathway defect, but C2 deficiency, combined defects, and assay artifacts can also produce a similar pattern. Repeat functional testing and component assays are needed ^[4].

The Second Step: Component Testing

If the functional tests show a break in the line, doctors then count the individual proteins (components) to help find the missing piece. In a typical complete C1s deficiency report, you might see ^[4]:

• C1s: Undetectable or extremely low.
• C1r and C1q: May also be low or at the bottom of the normal range. (This happens because these proteins stick together; if one is missing, the others can become unstable, though they are not uniformly low in every case) ^[4].
• C4 and C2: Often normal or slightly high.
• C3: Usually normal ^[4].

The Confusing Part: “False” Lows and Consumption

Two things can make a healthy person look like they have a deficiency, or make a deficiency hard to spot:

1. Lab Handling (Preanalytical Artifact)

Functional complement assays are extremely sensitive to collection, separation, transport, temperature, and freeze-thaw conditions ^[2]. If a sample is not handled appropriately, the proteins degrade, and the CH50 will drop to zero even in a perfectly healthy person ^[2][3]. The testing laboratory must determine whether a specimen is valid, and doctors often repeat these tests on a properly handled sample to be sure the result is real ^[5].

2. Acquired Consumption

If you have active lupus (SLE), your body might be “using up” its complement proteins faster than it can make them ^[6][7]. This is called consumption.

• Inherited Deficiency: The protein is missing because of a genetic defect. C3 and C4 are often normal, though this requires clinical interpretation ^[4].
• Acquired Consumption (SLE): The protein is missing because it was “used up” in a fight. In this case, C3 and C4 are typically low alongside C1, though they can sometimes remain normal during active disease ^[8][7].

The Final Proof: Genetic Testing

To confirm the diagnosis, a genetic test is performed to look at the C1R and C1S genes ^[4][9].

• Biallelic Mutations: To have the full complete deficiency, you typically need two pathogenic mutations—one from each parent ^[4].
• Sequencing: Most tests “read” the genetic code to find variants. A single variant or a Variant of Uncertain Significance (VUS) is not diagnostic on its own. A truncating variant only confirms the deficiency when its pathogenicity and biallelic status are established and correlated with protein results ^[4].

If sequencing is inconclusive but the doctor is still suspicious, they may order deletion/duplication testing to see if large chunks of the gene are missing ^[9].