Reliable Assessment of Iron Deficiency in Patients with Myeloproliferative Neoplasms through Fluorescence Spectroscopy-Based Measurement of Zinc Protoporphyrin (ZPP)

Iron deficiency markers are often unreliable: The most frequently determined parameter hemoglobin indicates only anemia, a late consequence of iron deficiency. Other parameters such as ferritin can be incorrect under certain health conditions, such as inflammatory diseases. Inflammation caused by myeloproliferative neoplasms (MPNs) significantly affects ferritin levels due to the body's inflammatory response. MPNs trigger the release of pro-inflammatory cytokines such as IL-6, which stimulates hepatic synthesis of ferritin, an acute-phase reactant. Elevated ferritin levels are therefore masking iron deficiency and complicating the clinical assessment of iron metabolism in MPN patients. This interplay between inflammation and iron metabolism necessitates careful evaluation of ferritin levels in MPN management. Zinc protoporphyrin (ZPP) is a compound found in red blood cells when heme production is inhibited by lead and/or by lack of iron. Instead of incorporating a ferrous ion to form heme, the immediate precursor of heme, protoporphyrin IX, incorporates a zinc ion, resulting in the formation of ZPP. Consequently, ZPP can be regarded as a marker of functional iron deficiency directly at the heme molecule level. The aim of this study was to evaluate ZPP as a bona fide marker of iron deficiency in MPN. Between September 2020 and June 2024 blood samples of MPN patients were analyzed. Data from 233 MPN patients and a cohort of 4420 individuals without diagnosis of MPN from the Study of Health In Pomerania (SHIP; SHIP-TREND-0, 2008-2012) were analyzed. Regarding the MPN diagnoses, 93 (40%) had been diagnosed with polycythemia vera (PV), 36 (15%) with essential thrombocythemia, 87 (37%) with myelofibrosis, and 17 (7%) with unclassifiable MPN. For quantitative ZPP measurement, fluorescence spectroscopy-based FIDscreen (prototype device, FerroSens GmbH) was used. ZPP levels < 40 µmol/mol heme were considered normal, 40-60 µmol/mol as indicator of mild, >60 µmol/mol as manifest, and >100 µmol/mol as severe iron deficiency. Standard descriptive statistics were used to describe the data. Associations between two continuous variables were determined using Pearson's correlation coefficient. In the control SHIP-study cohort a positive correlation between hemoglobin (HGB) and ferritin (Pearson correlation coefficient R = 0.27 (CI: 0.24, 0.29), p < 0.001) could be shown. In MPN patients, however, the correlation between HGB and ferritin was negative (R = -0.50 (CI: -0.63, -0.33), p < 0.001), thus limiting the use of ferritin as indicator for iron deficiency. One third of MPN patients had ZPP levels below 40 µmol/mol heme. ZPP levels were elevated in all MPN subgroups and values above 100 µmol/mol heme, indicating severe iron deficiency, were found in 45/233 (19%) patients. The highest ZPP levels were found in PV patients, where 25% (23/93) of patients had ZPP levels of 100 µmol/mol heme or higher. Data on phlebotomy-dependence were available for 53 patients with PV. In this subset of PV patients, mean ZPP values were significantly higher in patients who underwent phlebotomies than in patients who did not (101 vs. 64 µmol/mol heme, p = 0.035). Consistently, ZPP and HGB were negatively associated in MPN patients (R = -0.29 (CI: -0.42, -0.14), p < 0.001), whereas ZPP and ferritin did not show an association (R = -0.04 (CI: -0.23, 0.16), p = 0.709). We report on the suitability of ZPP as a reliable marker of iron deficiency in MPN patients. In contrast to the general population, ferritin is not a suitable iron deficiency marker in MPN patients, as many MPN patients may have elevated ferritin levels due to inflammatory processes. ZPP can therefore detect functional iron deficiency in MPN patients, which correlates closely with hemoglobin levels.