['Pegvisomant use in acromegaly negates the use of GH levels to monitor disease activity. To achieve antagonism, plasma concentrations must be approximately 1000-fold greater than GH which with the high homology between the peptides makes GH measurement a challenge when pegvisomant is present.', 'We investigated the effect of pegvisomant on GH measured using commercially available assays.', 'Pooled serum samples with GH concentrations <0.38, 3.85 and 7.69 microg/l were spiked with increasing pegvisomant concentrations (9000-494 000 microg/l). Samples were analysed by the Nichols Advantage, DPC Immulite 2000, Diasorin IRMA, Beckman Access Dxl, Tosoh AIA and Wallac Delfia assays.', 'With baseline GH <0.38 microg/l measured levels were <0.38 in all assays except Nichols, Diasorin and Beckman where GH peaked at 1.5, 9.6 and 17.7 micarog/l respectively at low pegvisomant concentrations, falling thereafter. With the other two samples, measured GH levels progressively fell with increasing pegvisomant concentrations, except the Beckman assay where an increase (30.8 microg/l) was seen at a pegvisomant concentration of 9000 microg/l; and Diasorin and Tosoh where smaller increases were seen at lower pegvisomant concentrations, levels gradually falling thereafter.'] Does pegvisomant interference in GH assays result in underestimation of GH levels?