Table 1. Summary of proposed methods for non-pathogen-specific immune monitoring in SOT recipients.

Characteristic	Serum immunoglobulins	Serum complement factors (C3, C4, MBL)	Peripheral blood lymphocyte sub-populations	Soluble CD30	iATP in CD4+ T cells (ImmuKnow assay)
Required sample	Serum	Serum	Whole blood	Serum	Whole blood
Assay	Nephelometry	Nephelometry or ELISA	Flow cytometry	ELISA	Quantification of iATP release in PHA-stimulated CD4+ T cells
Functional analysis	No	No	No	Yes	Yes
Advantages	Economical and easy to perform. Potential for replacement therapy with IVIGs	Economical and easy to perform. Potential for genotyping of mbl2 gene variants	Easy to perform (automatized methods)	Easy to perform. Commercial assay. Low volume of serum required (25 μl)	Only FDA-approved commercial assay. Highly standardized. Large volume of studies
Limitations	Lack of standardized cutoff values. No information on the functionality of the humoral response	Lack of standardized cutoff values. No information on the functionality of the complement system	Lack of standardized cutoff values. No information on the functionality of the cellular response	Only few studies on predicting infection with discordant findings	Only modest PPV and NPV in studies to date. Relatively high cost. Potentially biased by sample storage time

Abbreviations: ELISA, enzyme-linked immunosorbent assay; FDA, Food and Drug Administration; iATP, intracellular adenosine triphosphate; IVIGs, intravenous immunoglobulins; MBL, mannose-binding lectin; NPV, negative predictive value; PHA, phytohemagglutinin; PPV, positive predictive value; SOT, solid organ transplantation.