Summary
Background and aims
Determination of plasma volume (PV) is important in several clinical situations. Thus, patients with liver disease often have augmented PV as part of their sodium–water retention. This study was undertaken to compare PV determination by two indicators: technetium-labelled human serum albumin (99mTc-HSA) and iodine-labelled human serum albumin (125I-HSA), as the former may have advantages at repeated measurements and the latter is the classical gold standard.

Study population and methods
In 88 patients, (64 with liver disease, mainly cirrhosis, and 24 patients without liver disease), simultaneous measurements of PV were taken with 99mTc-HSA and 125I-HSA after 1 h in the supine position. Blood samples were obtained before and 10 min after quantitative injection of the two indicators. In a subset of patients (n = 32), the measurements were repeated within 1 h.

Results
In all patients, a close correlation was present between PV determined by the two indicators (r = 0·89, P<0·0001). In all, but twelve patients, a higher PV was obtained with 99mTc-HSA compared with 125I-HSA (P<0·0001). PV determined with 99mTc-HSA exceeded PV determined with 125I-HSA by 367 ml (5·2 ml kg−1) in liver patients as compared to 260 ml (3·5 ml kg−1) in patients without liver disease (P<0·05). The precision of repeated PV determination was 1·75% (coefficient of variation) with 99mTc-HSA and 1·71% with 125I-HSA (ns), and similar values were found in patients with and without liver disease.

Conclusion
The study demonstrates that 99mTc-HSA has the same precision as that of 125I-HSA. However, especially in patients with liver disease, 99mTc-HSA consistently overestimates the PV, most likely owing to indicator heterogeneity with subsequent fast removal from the circulating medium with a higher volume of distribution as the outcome.