The haematocrit to haemoglobin conversion factor: a cross-sectional study of its accuracy and application.
The definition of anaemia, a decrease of circulating red blood cell mass (1), does not change throughout life. However, the two main parameters commonly used interchangeably by medical practitioners to indicate anaemia, namely haemoglobin concentration (Hb) and haematocrit (Hct), are variable among different age groups and both genders. The World Health Organisation's (WHO) definition of anaemia uses Hb but not Hct in all its reports on the subject matter since 1968 (2). Four reference ranges are used to differentiate normal Hb from anaemia for each age group. In contrast, if both Hb and Hct are used for anaemia interpretation, eight reference ranges results for each age group and gender which further complicates the interpretation of anaemia, especially for those not in healthcare professions.
Furthermore, studies comparing Hb and Hct in detecting anaemia report that the use of Hb is more sensitive than Hct (3, 4). However, Hct holds one advantage over Hb due to being less expensive and more widely used than Hb (5), especially for screening purposes; although this advantage is diminishing as new and cheaper devices are being introduced in the markets for Hb measurement, e.g. HemoCue (5). In order to emphasize the use of Hb as the only parameter for anaemia assessment without losing the advantage of Hct measurements, the use of the three-fold conversion to equate the two measures has come into practice (6). To the best of our knowledge, most of the studies that have used the three-fold conversion method to derive the Hb have been exclusively done in malaria-endemic countries (6). Accurate determination of Hb level is an essential element in assessing the extent of anaemia and making a decision whether treatment is necessary or not (5). We believe that such an imminent decision needs a reliable, rapidly testable, and a widely available laboratory test. Accordingly, investigating the accuracy and applicability of the three-fold conversion use in other areas of the world is worthwhile, particularly in the middle East where a large proportion of the population lives in rural areas where laboratory measuring devices might not be always available.
The three-fold conversion, derived Hb = Hct/3, a ratio that combines two main laboratory parameters, can be used in the assessment and diagnosis of anaemia. This ratio can be used to provide a derived Hb value when the Hct is the only measured value available. Its use could provide results to diagnoses anaemia according to the WHO recommendations leaving just four reference ranges for healthcare practitioners to use in daily practice. The aim of our retrospective study was to evaluate the accuracy and to determine factors that might affect the applicability of conversion of the Hct to provide a derived Hb for the diagnosis of anaemia in middle-eastern populations, particularly in situations where a measured Hb value might not be readily available.
MATERIALS AND METHODS
We retrospectively obtained the haematology data for 1695 patients from the University of Jordan Hospital computer system, including general demographics (i.e. age, gender), and kidney function tests results. Each patient had to have had a complete blood count (CBC) and kidney function tests within a previous 24-hour period and not to have been diagnosed with or suspected to have malaria. In addition, pregnant women were excluded. The three-fold conversion of the Hct to obtain the Hb was conducted and the absolute difference between the measured Hb and the derived Hb was calculated to provide the "Delta Hb" according to the following formula: Delta Hb = (measured Hb-(Hct/3).
We compared the derived Hb for each patient to the corresponding measured Hb, then calculated the sensitivity, specificity, and positive predictive value of the derived Hb for the purposes of anaemia detection using the following formulae:
Sensitivity = true positive / (true positive + false negative) Specificity = true negative / (false positive + true negative) Positive predictive value = true positive / (true positive + false positive)
* True negative; measured and derived Hb indicates normal.
* False positive; derived Hb indicates anaemia while measured does not.
* True positive; measured and derived Hb indicates anaemia.
* False negative; measured Hb indicates anaemia while derived value does not.
Normal and anemic Hb levels were determined based on the haemoglobin reference ranges presented in WHO report (7). The following reference ranges were considered for aneamia diagnosis:
* Children 6-59 months: below 110 g/L.
* Children 5-11 years: below 115 g/L.
* Children 12-14 years and women [greater than or equal to] 15 years: below 120 g/L.
* Men [greater than or equal to] 15 years: below 130 g/L.
The Siemens' high-volume haematology analyzer (ADVIA[R] 2120i System with Autoslide) was used to provide the patients' Hb and Hct.
Statistical analysis of the data was done using SPSS for Windows release 21.0 (SPSS Inc., Chicago, IL, USA). Descriptive statistics were used to describe our sample populations, including mean and standard deviation for continues variables, and proportions for other types of variables. We used the independent sample t-test to study gender differences in both the Hb and Hct values. Due to a nonlinearity relationship we used Spearman's test to study the relationship between Hb and Hct with age, creatinine, and the blood urea nitrogen to creatinine ratio (BUN/Cr). We also used Spearman's to study the relationship between the difference between the measured and derived Hb (Delta Hb) with Hct, age, creatinine, and the BUN/Cr) ratio. A p-value of <0.05 was considered statistically significant. This study was approved by our institutional ethical committee and conducted in accordance with latest update (2013) of the declaration of Helsinki.
A total of 1695 patients were included, these were 849 (50.1%) male and 846 (49.9%) female with a combined mean age of 56.7 years ([+ or -] 20.48). The mean Hb and Hct values were 128 g/L ([+ or -] 21) and 39.1% ([+ or -] 6.3), respectively. The gender difference for Hb was significant (p value <0.001), as mean Hb for males was 134 g/L ([+ or -] 22.4) and for females was 122 g/L ([+ or -] 17.6). The gender difference for Hct was also significant (p<0.001), as mean Hct for males was 40.69% ([+ or -] 6.66) and for females was 37.37% ([+ or -] 5.49). On Spearman's test, both the Hb and Hct were significantly correlated with age, creatinine, and BUN/Cr ratio, as shown in Table 1.
63% of the sample had a derived Hb higher than the measured Hb and 37% had a measured Hb higher than the derived Hb. The Delta Hb value was significantly correlated to Hct levels (p= 0.001) with a Spearman's coefficient of 0.08 (Figure 1). There was no significant correlation between Delta Hb and patient's age, creatinine, or BUN/Cr ratio (p values of 0.08, 0.32, and 0.41, respectively).
Based on the WHO definition of anaemia, 33.6% (a total of 563 patients) had a measured Hb below the reference range for anaemia, whereas 32.5% (a total of 544 patients) were anaemic using the their derived Hb value. Using the derived Hb values the method provided 85.94% sensitivity, 94.50% specificity and a positive predictive value of 88.8%.
Worldwide, the laboratory gold standard method used to screen for anaemia is the Hb and/or Hct levels (8). In 2001 a WHO report recommended the use of the Hb value in clinical surveys claiming that the Hct value only added complexity, thus making results more challenging for decision-makers to interpret in the classification of anaemia (9). The use of Hb as a parameter to assess potential anaemia is sometimes not possible in the clinical setting where the Hct value is the only means available for assessment of anaemia. This might be also of particular importance for assessment and follow up of patients in regions where malaria is common. Under such circumstances a simple conversion formula is needed to provide Hb values, such as the three-fold conversion formula (2). This study has shown that the well-known three-fold conversion of the Hct to derive a Hb value provided results that are not affected by the patient's age, gender, hydration status as measured by the BUN/Cr ratio and kidney function. We also showed that the Hb value derived from the Hct is able to detect anaemic patients with 85.94% sensitivity, 94.50% specificity and a positive predictive value of 88.8%.
More importantly, we found that the accuracy of the derived Hb value increases at lower Hct levels. Delta Hb, the difference between measured and derived Hb values, is positively correlated to Hct levels, therefore, Delta Hb declines at lower Hct levels. This implies that the accuracy of the three-fold conversion increases as Hct levels decrease, making it a reliable measure to calculate the Hb for patients within the anaemia range; such as upon assessment of patients from geographic regions and age groups were anaemia is common, e.g. malaria-endemic regions and elderly age group, respectively. In another study a similar association was found in malaria-infected children where the difference between the measured and the calculated (derived) Hb was found to decrease as the anaemia worsened in severity (10).
Unexplained anaemia, where no specific aetiology can be identified, is common and accounts for one third of anaemia cases in elderly populations (11). Thus, it is worthwhile to investigate the accuracy and applicability of the three-fold conversion in this group. Recent studies suggest age-related progressive erythropoietin resistance leads to a decline in RBC production and RBC number (11). Additionally, microcytic anaemia (low mean cell volume, MCV) is common in the elderly. As many automated haematology analysers calculate Hct by multiplying the red cell count by the MCV (5, 1), a reduction in either or both of these indices may affect the Hct and consequently, the derived Hb and Delta Hb. Our results showed that Delta Hb is not correlated to age, thus the derived Hb can be reliably used in assessment of elderly populations.
A study by Hsu et al. showed that the loss of kidney function leads to a decrease in Hct levels and the development of anaemia (12). In our study, we found that an increase in creatinine (impaired kidney function) was associated with a decrease in both Hb and Hct. Furthermore, loss of kidney function did not affect Delta Hb, thus, it might not affect the accuracy of using derived Hb in those patients. This makes the derived Hb a reliable parameter to assess Hb levels in patients with impaired kidney function. Our findings also showed that Delta Hb is independent of a patient's hydration status (as measured by the BUN/Cr ratio), gender, or age.
The widespread use of the Hb value to reflect the blood oxygen carrying capacity and indirectly anaemia has multiple advantages. Firstly, it is easier for multidisciplinary health practitioners to communicate with each other using a standard parameter that can be derived irrespective of the devices being used in the clinical setting. Second, Hb is identified by the WHO as the parameter used principally for defining anaemia among different age groups, and so it is necessary to calculate the Hb when only the results of the Hct levels are available. Third, having a single universal parameter in research will make published research more comprehensible to a wider group of readers, so that studies that utilize only Hct in its results can be compared with studies only utilizing Hb by virtue of the threefold conversion.
Interestingly, most of the studies that have used the three-fold conversion method to derive the Hb have been conducted in malaria-endemic countries. Recent studies on malaria-infected children show that the gap between the derived and measured Hb values decreases as the severity of anaemia increases (6, 10). On the other hand, Carneiro et al. demonstrated that this gap increases in malaria-infected children below the age of 5 years, where the drop in the derived Hb levels does not go hand in hand with the actual drop in their measured Hb levels (6). Thus, the use of derived Hb for evaluation of children below age of 5 might lead to underestimation of the prevalence of anaemia in malaria-infected children (6).
Worth mentioning is that a recent study suggests another conversion formula that is more reliable for use in malaria-endemic regions; Hct = 5.62 + 2.6 * Hb. This formula has since been found to be comparable to the three-fold conversion but with the latter being more practical and simplified by comparison (13).As a contribution for to the epidemiologic data available in the literature regarding mean Hb levels of different populations worldwide, we compared the mean Hb of our middle-Eastern sample to some of the mean values available in literature for other populations. Noticeably, by comparing the mean Hb of our sample and that of the malaria-endemic region (sub-Saharan Africa) in the Carneiro et al. study (6), a difference of 23 g/L between the mean Hb values of the two regions was found (128 g/L in the middle East compared to 105 g/L in a malaria endemic region). Conversely, generally higher Hb levels were noted among American population for both whites (149 g/L in males, 135 g/L in females) and African Americans (145 g/L in males, 127 g/L in females) (14).
There are some limitations for our study that need to be taken into account. These include the inability to assess the hydration status of our patients clinically, only the BUN/Cr ratio was used to reflect the hydration status of the patients. Moreover, as this study was retrospective, the confounding effect of some other factors, such as polycythemia, haemoglobinopathies, or blood sample haemolysis, could not be assessed. We recommend that future studies be held in other populations to assess the validity of the three-fold conversion in different regions of the world, and to take the afore mentioned potential interferences into account.
WHO reports recommend the use of the Hb alone for the interpretation of anaemia. However, not all facilities have access to Hb measurement, instead relying only on Hct. We recommend the use of the three-fold conversion to derive Hb, so that the derived Hb can be used as a reliable parameter between healthcare practitioners. The derived Hb can be conveniently used for the assessment and follow up of a large group of patients, as its accuracy increases at lower Hct levels, and for being not affected by patient's age, gender, hydration status, or kidney function. The three-fold conversion is also reliable for assessment of anaemia in elderly patients and in patients with impaired kidney function.
The authors thank the Deanship for Research at the University of Jordan for their guidance regarding the statistical analysis.
Nosaiba Al-Ryalat, MD, Associate Professor
Saif Aldeen AlRyalat, MD, Clinician
Lna Waleed Malkawi, MD, Clinician
Hana Abu-Hassan, MD MBBS MRCGP JBFM, Assistant Professor
Osama Samara, MD, Associate Professor
Azmy Hadidy, MD FRCR, Professor
Department of Radiology and Nuclear Medicine and Department of Family Medicine, Faculty of Medicine, University of Jordan, Amman, Jordan
Correspondence: Saif Aldeen Saleh AI Ryalat.
P.O. Box no. 1669 Tela Al Ali 11953 Amman, Jordan.
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The Editors have identified three author errors in Figures 1a and 1b at the time of going to print. Figure 1a, cHg for the key should read cHb and on the x-axis cHg should also read cHb. In Figure 1b on the y-axis cHg should read cHb.
Nosaiba Al-Ryalat, Saif Aldeen AlRyalat, Lna Waleed Malkawi, Hana Abu-Hassan, Osama Samara and Azmy Hadidy
University of Jordan, Amman, Jordan
Caption: Figure 1a. Illustrates the mean measured and derived hemoglobin (Hb) values at different hematocrit (Hct) levels. The difference between derived Hb and measured Hb (mHb) is labelled in red as (Delta Hb).
Caption: Figure 1b. Shows Delta Hb (Red labels in figure 1a). Delta Hb increases at higher Hct levels, thus widening the gap between measured and derived Hb. The greater the gap between measured and derived Hb (Delta Hb), the less accurate the three-fold conversion becomes.
Table 1. Correlation between both H and Hot and age, creatinine, and BUN/Cr ratio. Factor Hb/Hct rho P Age Hb -0.213 <0.001 Hot -0.188 <0.001 Creatinine Hb -0.098 <0.001 Hot -0.076 0.002 BUN/Cr Hb -0.187 <0.001 Hot -0.181 <0.001 Rho = Spearman's correlation coefficient. BUN/Cr = blood urea nitrogen to creatinine ratio.
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|Title Annotation:||ORIGINAL ARTICLE|
|Author:||Ryalat, Nosaiba Al-; AlRyalat, Saif Aldeen; Malkawi, Lna Waleed; Abu-Hassan, Hana; Samara, Osama; Ha|
|Publication:||New Zealand Journal of Medical Laboratory Science|
|Date:||Apr 1, 2018|
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