IgG4ward! Question of the Week: “Which blood tests should be monitored in IgG4-RD, and how often should they be checked?”

We are pleased to share another edition of the IgG4ward! Question of the Week (QoW) series! This week's QoW and its answer are below. We invite you to join the Online Community and share any follow-up questions or comments you might have about the QoW there.

“Which blood tests should be monitored in IgG4-RD, and how often should they be checked?"

A handful of simple blood tests, when measured regularly, can be extremely helpful in monitoring disease activity in IgG4-RD. The first of these, not surprisingly, is IgG4.

IgG4

The serum (i.e., blood) IgG4 level is the best biomarker in the majority of patients. In addition, the higher it is before treatments starts, the better it tends to be at reflecting the state of disease activity.

Here is a key point:

The direction in which the serum IgG4 level is moving is more important than whether it is elevated or not. By this, I mean that:

• If the IgG4 level is lower than it was at the previous measurement, that is reassuring that the disease is coming under control (or remains under control).

• On the other hand, if the level is rising – particularly if there is a substantial jump in the IgG4 level compared to the previous value – then that may indicate that the disease is becoming active again and that re-treatment at some point in the near future should be considered.

Other tests can also be helpful, though they are usually less for most patients than the serum IgG4 level. These other tests are:

IgG1 and IgE

IgG1 and IgE are antibodies, just as IgG4 is (We will talk more about what an antibody is in our upcoming Conversation series – stay tuned). The levels of these antibodies tend tend to rise when the underlying disease is active. In this way, they usually reflect the direction of the IgG4 level, though the IgG4 is typically more reliable. There can be some “noise” in the IgE level because it can also be affected by co-existing allergic issues that may have nothing to do with IgG4-RD.

C3 and C4

C3 and C4 are not antibodies. Rather, they are proteins that form part of another key component of the immune system, the complement system (the C in C3 & C4 stands for complement). The C3 and C4 measurements tend to become LOW, not high, when IgG4-RD is active. However, the serum C3 and C4 values reflect disease activity reliably in only about 40% of patients. And in general, when the C3 and C4 values are low – suggesting active disease – the IgG4 value is also usually rising (suggesting the same thing).

In summary, the serum IgG4 value is the most reliable biomarker for most patients with IgG4-RD. The information from the IgG4 level in the blood – and the direction in which it is moving – is supplemented by information from the IgG1, IgE, and complement (C3 & C4) levels.

How often should these blood tests be checked?

And with regard to Part 2 of the question: how often should these blood tests be checked? The answer is pretty simple. Every three or four months suffices very well for most patients.

Please let us know if these answers are helpful (and if they raise more questions). If you have a follow up question or comment, please share your feedback in the IgG4ward! Online Community.