# Carbon monoxide rebreathing method is a reliable test to evaluate the red cell mass in polycythaemia

Nada Maaziz, Marjolaine Georges, Damien Basille, Matthieu Gallet, Betty

Gardie, Momar Diouf, Loïc Garçon, François Girodon

► To cite this version:

Nada Maaziz, Marjolaine Georges, Damien Basille, Matthieu Gallet, Betty Gardie, et al. Carbon monoxide rebreathing method is a reliable test to evaluate the red cell mass in polycythaemia. British Journal of Haematology, 2024, 204 (2), pp.706-709. 10.1111/bjh.19169. hal-04383116

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Submitted on 23 Jun 2025

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# Carbon monoxide rebreathing method is a reliable test to evaluate the red cell mass in polycythaemia

Elevated haemoglobin (Hb) and/or haematocrit (Ht) values are frequently observed in routine medical practice, raising the question of erythrocytosis. Absolute polycythaemia is defined as a red blood cell mass (RCM) surpassing 125% of the theoretical value,¹ assessed by isotopic measurement. In routine practice, Hb and Ht are used as surrogate markers in the diagnosis of polycythaemia. While it is accepted that absolute polycythaemia is certain for Ht >56% in women and 60% in men in the absence of dehydration,² several series have demonstrated discrepancies between Hb, Ht and RCM for lower values.³⁻⁵

The 'optimized carbon monoxide re-breathing method' (oCOR), based on haemoglobin's affinity for CO, has been used to evaluate RCM since the end of the 19th century,⁶ and numerous seminal studies validated its usefulness and relevance in this field in the early 20th century.⁷⁻⁹

The recent abandonment of isotopes in medicine has brought the oCOR method back into use, particularly thanks to the adaptation of more efficient methods in athletes and the use of more modern equipment.¹⁰,¹¹

Thus, the possibility of using a reliable measurement of RCM as an alternative to isotopic labelling would be an undeniable help in the diagnosis of polycythaemia.

A recent pilot study of seven PV patients reported 100% sensitivity for the detection of polycythaemia.¹² However, no study has compared RCM values obtained with the oCOR method and isotopic labelling in the diagnosis of polycythaemia. Here, we present the result of a prospective bicentric study comparing RCM obtained by the oCOR test and by isotopic measurement in a population of 42 patients referred to a haematologist for suspicion of polycythaemia.

Patients suspected of having polycythaemia were prospectively included in a single-arm, non-randomized study if they met the inclusion criteria listed on Beta.clinicaltrials.gov (# NCT03008642). Ethical approval was granted by the Ile de France III Ethics Committee, and written informed consent was obtained from all participants. RCM determination was performed as recommended by the International Council for Standardization in Hematology.¹³ The parameters of the oCOR method were measured according to the studies previously described: Arterialized blood samples were harvested to immediately determine the percentage of HbCO; Hb and Ht were measured at the same time; a respiratory function test

was performed for each patient as well as a DLCO measurement confirming the absence of respiratory disorders.¹¹,¹⁴

We ensured that the patient did not have phlebotomy between the two measurements and checked for any abnormality that could explain a variation in RCM.

A total of 42 patients were included in the study. We were unable to perform the oCOR measurement for two patients: one had smoked before the examination, and one was ergonomically unable to use the spirometer. The isotopic measurement could not be performed in a third patient because of difficulty reinjecting labelled RBCs. Finally, 39 patients with a median age of 57 years completed the study evaluating the RCM using both techniques, of whom three patients had JAK2-positive PV. Splenomegaly was observed in three patients (2 with PV). The results of the two techniques were consistent in 31 (79.5%) patients. For four patients, the oCOR test underestimated the RCM compared to the isotopic measurement, while for four others, the value obtained by the oCOR test was overestimated (Figure 1). Overall, compared to the isotope technique, the oCOR had a sensitivity of 84% and a specificity of 71% in our cohort.

This comparison study of the two methods of evaluation of RCM concerns, to the best of our knowledge, the largest series of patients suspected of polycythaemia. In our study, the failure rate (2.4%) for the oCOR test was much lower than previously reported (10%),¹² but similar to another,¹⁵ highlighting that adequate management allows the technique to be validated in the vast majority of patients. When comparing the RCM results obtained with the two methods, we found that they were consistent in nearly 80% of patients, underlying that the oCOR method is reliable. A slight discrepancy was noted in eight patients: using the oCOR test, the RCM was overestimated in four patients, whereas it was underestimated in four other patients, contrary to previous results.⁷ Our first hypothesis was that an underlying lung disease would modify the CO uptake. However, the respiratory function tests performed before the oCOR test ruled out any respiratory problems. The BMI index was not correlated with the risk of discrepancy between the two methods. Neither was the presence of splenomegaly, seeing that RCM was concordant between the two methods in two of the three patients with splenomegaly. When we focused on the four false-negative patients, that is with an underestimation of RCM assessed by the oCOR test, three had values close to the 125% threshold (117%, 119% and 122%).
<figure><img src="image_2.png" /><figcaption>**FIGURE 1** Correlation of red cell mass (RCM) values measured by the isotopic method (RCM #I) and carbon monoxide rebreathing (RCM #CO). The vertical axis (y) represents the values of RCM measured by the isotopic method, and the horizontal axis (x) represents the values of RCM measured by carbon monoxide rebreathing. Green dots represent consistent RCM values, whereas red dots represent discordant RCM values.</figcaption></figure>

One could imagine that the threshold using the oCOR method for polycythaemia diagnosis might be lower than 125%. However, this hypothesis would require a higher number of patients to confirm. We therefore wonder whether defining a 'grey zone' of uncertainty between 115% and 125%, which would require a second evaluation by the oCOR test, would be helpful. Given the speed, low cost and accessibility of the technique, which can be performed in any pneumology department, checking the values close to the threshold would perhaps make it possible to limit the number of discrepancies with the reference technique.

Such a technique would potentially be useful not only at diagnosis but also during the follow-up of PV patients with splanchnic thrombosis. Due to the frequent voluminous splenomegaly in these pathological situations, Ht is a poor indicator of RCM reduction. Monitoring based on a fast RCM measurement would probably allow better control of erythrocyte expansion.

In conclusion, our preliminary results confirm the reliability of the oCOR method for the evaluation of RCM in suspected polycythaemia. This technique is rapid, inexpensive

and easily done. Moreover, it can be repeated and could be used to monitor treatment. Our results will have to be confirmed on larger cohorts, in particular through a prospective multicentre study that we are currently setting up.

## AUTHOR CONTRIBUTIONS

François Girodon, Nada Maaziz, Marjolaine Georges and Loïc Garçon wrote the manuscript; Marjolaine Georges and Damien Basille performed oCOR analyses; François Girodon and Loïc Garçon recruited patients; Nada Maaziz, François Girodon, Momar Diouf and Loïc Garçon performed data analyses; Loïc Garçon and François Girodon designed the study; Betty Gardie and Damien Basille revised the manuscript.

## ACKNOWLEDGEMENTS

This study was supported by grants from Novartis. The authors thank Suzanne Rankin for revising the English manuscript.

## FUNDING INFORMATION

Not applicable.
## CONFLICT OF INTEREST STATEMENT

The authors declare no competing financial interests.

## DATA AVAILABILITY STATEMENT

Reasonable requests for the data presented in this paper can be made to the corresponding author, Pr. François Girodon.

## ETHICS STATEMENT

Ethical approval was granted by the Ile de France III Ethics Committee.

## PATIENT CONSENT STATEMENT

Written informed consent was obtained from all participants.

## PERMISSION TO REPRODUCE MATERIAL FROM OTHER SOURCES

All content is original.

## CLINICAL TRIAL REGISTRATION NUMBER

NCT03008642.

Nada Maaziz<fnref n="1" />,<fnref n="2" />

Marjolaine Georges<fnref n="3" />

Damien Basille<fnref n="4" />,<fnref n="5" />

Matthieu Gallet<fnref n="6" />

Betty Gardie<fnref n="7" />,<fnref n="8" />,<fnref n="9" />

Momar Diouf<fnref n="10" />

Loïc Garçon<fnref n="11" />,<fnref n="12" />

François Girodon<fnref n="2" />,<fnref n="9" />,<fnref n="13" />,<fnref n="14" />

## Correspondence

François Girodon, Service d'Hématologie Biologique, Hôpital du Bocage, Centre Hospitalier Universitaire (CHU) de Dijon, 2 rue Angélique Ducoudray, Dijon, France.

Email: francois.girodon@chu-dijon.fr

Nada Maaziz and Marjolaine Georges contributed equally to this work; Loïc Garçon and François Girodon contributed equally to this work.

## ORCID

François Girodon https://orcid.org/0000-0003-3151-1068

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<footnote n="1">¹Service de Génétique Chromosomique et Moléculaire, Pôle Biologie, Centre Hospitalier Universitaire (CHU) de Dijon, Dijon, France</footnote>

<footnote n="2">²Inserm U1231, Université de Bourgogne, Dijon, France</footnote>

<footnote n="3">³Service de Pneumologie, Centre Hospitalier Universitaire (CHU) de Dijon, Dijon, France</footnote>

<footnote n="4">⁴Service de Pneumologie, Centre Hospitalier Universitaire (CHU) d'Amiens-Picardie, Amiens, France</footnote>

<footnote n="5">⁵UR 4294 AGIR, Université Picardie Jules Verne, Amiens, France</footnote>

<footnote n="6">⁶Unité de Radiopharmacie et/ou Service Pharmacie, Centre Georges François Leclerc (CGFL), Dijon, France</footnote>

<footnote n="7">⁷Université de Nantes, CNRS, INSERM, l'institut du Thorax, Nantes, France</footnote>

<footnote n="8">⁸Ecole Pratique des Hautes Etudes (EPHE), Université PSL, Paris, France</footnote>

<footnote n="9">⁹Laboratoire d'Excellence GR-Ex, Paris, France</footnote>

<footnote n="10">¹⁰Direction de la Recherche et de l'innovation, Centre Hospitalier Universitaire (CHU) d'Amiens-Picardie, Amiens, France</footnote>

<footnote n="11">¹¹Service d'Hématologie, Centre Hospitalier Universitaire (CHU) d'Amiens-Picardie, Amiens, France</footnote>

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<footnote n="14">¹⁴Membre du France Intergroupe des Myéloprolifératifs (FIM), Paris, France</footnote>
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