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Usefulness of P50,std for the Diagnostic Work-up of Patients with Erythrocytosis
J Lab Med Qual Assur 2018;40:46-49
Published online March 31, 2018
© 2018 Korean Association of External Quality Assessment Service.

Sang-Yong Shin

Department of Laboratory Medicine, Seoul Metropolitan Dongbu Hospital, Seoul, Korea
Correspondence to: Sang-Yong Shin Department of Laboratory Medicine, Seoul Metropolitan Dongbu Hospital, 124 Muhak-ro Dongdaemun-gu, Seoul 02584, Korea Tel: +82-2-920-9236 Fax: +82-2-920-9368 E-mail: hypocra@naver.com
교신저자: 신상용 우)02584 서울시 동대문구 무학로 124, 서울특별시 동부병원 진단검사의학과 Tel: 02)920-9236, Fax: 02)920-9368, E-mail: hypocra@naver.com
Received December 18, 2017; Revised January 18, 2018; Accepted January 22, 2018.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
 Abstract

High oxygen-affinity hemoglobin (Hb) variants and a 2,3-diphosphoglycerate (2,3-DPG) deficiency could cause congenital (familial) erythrocytosis. High oxygen-affinity Hb variants and a 2,3-DPG deficiency might result in low tissue oxygen tension left-shifted oxygen dissociation curves and reduction in the standard P50 value (P50,std, oxygen tension at which haemoglobin is 50% saturated). Hence, the P50,std value is considered while formulating diagnostic strategies for erythrocytosis. In this study, we established a reference range for P50,std using an International Federation of Clinical Chemistry and Laboratory Medicine-approved equation (Hill’s equation) for individual single venous/arterial blood samples. Blood gas analysis results of 243 samples with oxygen saturation ranging from 40%–90% (Hb<16 mg/dL) were selected. The reference range of P50,std was in the 2.5th–97.5th percentile, and was 25.9–27.3 mm Hg. Hill’s equation is a simple approved method for evaluating the P50,std values. Only a single sample of venous or arterial blood and a blood gas analyser are required to obtain the P50,std. Our study provides a useful tool for the diagnostic work-up of patients with erythrocytosis.

Keywords : Hill’s equation, P50, Polycythemia
Body
igh oxygen-affinity hemoglobin (Hb) variants and a 2,3-diphosphoglycerate (2,3-DPG) deficiency could result in low tissue oxygen tension left-shifted oxygen dissociation curves and reduction in the P50 value (the oxygen tension at which haemoglobin is 50% saturated) [1-3]. The P50 value has been used as a diagnostic screening tool for erythrocytosis [4]. It can be calculated from a single blood gas analysis result. Hill’s equation is derived from a mathematical model for the haemoglobin-oxygen dissociation curve [5]. The standard P50 (P50,std) has been proposed by the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) to minimise errors in the determination of the P50 value [6,7]. P50,std is defined as the P50 obtained when pH=7.40, PCO2=5.33 kPa (40 mm Hg), and temperature=37.0°C [6,7]. P50 and P50,std are calculated using the following equations [7]:

logP50=logPO2log[SO2/(1SO2)]/nHill

where nHill=2.7 (Hill’s equation constant), PO2 is the oxygen tension (mm Hg), and SO2 is oxygen saturation.

logP50.std=logP50+ΦH(7.4pH)+ΦH(log40logPCO2),

in which ΦH: proton Bohr factor, -0.43 and ΦC: carbamate Bohr factor, 0.05

In this study, we have established the reference range of P50,std using the IFCC approved equation (Hill’s equation) for the results of a single venous/arterial blood gas analysis [2,7]. The results of blood gas analysis for samples (venous or arterial sample) obtained between 1st October 2016 and 31st December 2016 were selected. Results for samples showing oxygen saturation between 40% and 90% and Hb levels ≤16 mg/dL (no erythrocytosis) were included in this study [5,7]. An oxygen saturation range of 40%-90% was selected because the relationship between PO2 and SO2 in this range was better, as indicated by the calculations made using Hill’s equation. Blood gas analysis was performed using ABL 80 Flex (Radiometer, Westlake, OH, USA). All tests were performed at 37°C±0.2°C, according to the manufacturer’s instructions.

Results from 243 individuals (155 males and 88 females) were included in this study. According to the Clinical and Laboratory Standards Institute guidelines, the reference range of P50,std was in the 2.5-97.5 percentile (25.9-27.3 mm Hg) [8]. There were no significant differences between the P50,std values for males and females (26.6 mm Hg in male versus 26.5 mm Hg in female, P=0.101). The P50 value was slightly higher than the P50,std value (Fig. 1A). Using the Kolmogorov-Smirnov (K-S) test, it was observed that P50,std showed a normal distribution (K-S value=0.458, P=0.985) (Fig. 1B).

Figure 1.

(A) Bland-Altman plot of the P50 and P50,std values, (B) Distribution of the P50,std. P50 value is slightly higher than the P50,std value (A). A normal distribution (Kolmogorov-Smirnov value=0.458, P=0.985) is observed for the P50,std. The reference range for the P50,std is in the 2.5th–97.5th percentile (25.9–27.3 mm Hg) (B). Abbreviation: P50,std, standard P50.


During the initial diagnostic work-up of patients with erythrocytosis, individuals with acquired primary erythrocytosis (polycythaemia vera with JAK2 mutation and low serum erythropoietin [EPO] level) and acquired secondary erythrocytosis (pulmonary, renal, cardiac, and others) must be excluded based on their medical histories, results of the JAK2 mutation study, and serum EPO levels. A low serum EPO level with absence of JAK2 mutation is suggestive of primary familial congenital polycythaemia associated with an EPOR mutation. In individuals with a normal or high serum EPO level, determination of the P50,std value is recommended to rule out the presence of high oxygen-affinity Hb variants or 2,3-DPG deficiency [4]. In a previous report, 4 out of 102 patients with isolated erythrocytosis exhibited low P50 values (three Hb variant patients and one 2,3-DPG deficiency patient) [9]. Based on this diagnostic strategy, there has been a report on a Korean patient with a high oxygen-affinity Hb variant (Hb Heathrow; isolated erythrocytosis, normal EPO level, and low P50) [10].

Hb variants are frequently reported in Korea, while 2,3 DPG deficiency is relatively rare [11-13]. Two patients with Hb variants have been identified among 38 Korean patients with isolated erythrocytosis [12]. Therefore, the P50,std value could be useful in the diagnostic work-up of patients with idiopathic erythrocytosis.

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