Pratt, Kevin

Abstract Objective Biomagnetic liver susceptometry (BLS) is a noninvasive method to quantify liver iron concentration (LIC). Here we report our findings from a prospective study which validates in vivo LIC from a SQUID biosusceptometer by in vitro LIC in fresh tissue and paraffin-embedded biopsies from patients at risk for iron overload.Materials and Methods LIC was measured by BLS and biopsy. LIC by biopsy were measured in 40 dry weight fresh tissue and paraffin-embedded liver biopsy samples. LIC from biopsies and total iron scores from histology were compared to biosusceptometry. In addition, the wet-to-dry weight ratio was determined.Results Liver iron concentrations measured by BLS and in 40 fresh tissue biopsies were related by a factor of 6.0 ± 0.2 (r2 = 0.88). Similar results were obtained from comparisons with deparaffinized biopsies (6.6±0.3, r2=0.87) and histology (6.7±1.3, r2=0.47). In contrast, a mean wet-to-dry weight ratio of 4.1 ± 0.7 was achieved from biopsies immediately weighed after the biopsy procedure.ConclusionLIC derived from two independent measures, the historical biopsy gold standard and biosusceptometry, were highly correlated. When comparing biosusceptometry with wet weight biopsies, the liver tissue sample size is critical.

Abstract The “Optimal Liver Iron” range in iron overloaded patients with thalassemia has been established by Olivieri and Brittenham (Blood, 1997) as 1–2.2 mg/g wet weight liver using direct measurement or calculated as 3.2 to 7 mg/g dry liver weight based on a 3.33 correction factor for body water. This report describes the relationship between liver iron concentration (LIC) measured by SQUID biosusceptometry (BLS, wet weight) compared to liver biopsy (dry weight), in patients with thalassemia (THAL) or sickle cell disease (SCD). A total of 38 chronically transfused patients (THAL n=19, SCD n=19) were prospectively assessed for LIC measured by BLS and liver biopsy within 2 month of each other. Most BLS results were submitted to data repository before receiving iron concentration from liver biopsy. LIC was measured by a low temperature SQUID biosusceptometer system (Ferritometer®) under the standardized Hamburg-Torino-Oakland protocol. Iron in fresh tissue and paraffin embedded liver biopsy samples was measured at Mayo clinics by Inductively Coupled Plasma Mass Spectrometer (ICP-MS). Subjects ranged in age from 5 to 40 years (median: 18 y). Median LIC (n=38) measured by SQUID was 1777 (365–4883) [mg/g wet weight], median LIC by biopsy (fresh tissue) was 10861(1854–32864) [mg/g dry weight]. After excluding the subjects with BMI>30 kg/m2 (n=5), the Spearman rank correlation between wet weight LIC assessed by BLS and dry weight LIC from fresh tissue sample measured by ICP-MS was highly significant (RS=0.90, p<0.0001). No significant difference between fresh tissue and paraffin embedded LIC values was observed. The agreement between BLS and fresh tissue biopsy was tested by Bland-Altman plots. After normalization, no significant proportional or zero bias was visible. The 95% limits of agreement between SQUID-BLS and biopsy LIC values were found between −56% (or –6733 μg/g) and 48% (or 7027 μg/g). This kind of agreement is similar to the results achieved with MRI-R2 by St. Pierre et al (Blood, 2005)(–56% and 50%), however, BLS is a totally independent method relying only on the specific magnetic susceptibility of the hemosiderin-ferritin iron complex. The conversion factor between the two methods (LIC by fresh tissue biopsy vs. LIC by BLS) obtained from a weighted linear regression with zero intercept (Marquardt algorithm) was 6.1 ± 0.3 (R2 = 0.86–0.88, cn2 = 2.3–2.6). Using this conversion factor, the recommended “optimal range” for liver iron in patients with thalassemia major should be considered as 6–13.2 mg/g dry weight liver rather than the generally accepted 3.2–7 mg/g dry weight liver.