Hemoglobin oxygen dissociation (P50) in bronchopulmonary dysplasia.

A study was devised to determine the P50 in infants with bronchopulmonary dysplasia (BPD). Other factors such as red blood cell 2,3-diphosphoglycerate (2,3-DPG) level proportions of adult (HbA) to fetal (HbF) hemoglobins which could affect P50 were also measured. Fourteen infants with clinical and radiological signs of BPD with a mean post-conceptional age of 42.2 +/- 4.7 weeks born at a mean gestational age of 29.3 +/- 2.0 weeks were evaluated. The percentage of HbF determined in 8 infants was 40.1 +/- 20.3% and the mean 2,3-DPG concentrations was 13.1 +/- 2.2 mumol/g Hb. The P50 was 25.1 +/- 2.7 mm Hg (range 18-29.5 mm Hg). When a HbO2 curve was established based on a large volume of blood consisting of adult blood and newborn cord blood mixed to attain a P50 of 25.1 mm Hg, the PaO2 at 90% O2 saturation was 52 mm Hg. Since there can be a wide range in HbO2 in infants with chronic BPD, pulse oximetry remains the most prudent method of monitoring oxygen therapy in BPD infants.     

OXYGEN AFFINITY OF HAEMOGLOBIN AND RED CELL 2,3-DIPHOSPHOGLYCERATE IN CHILDHOOD DIABETES

ABSTRACT: Ditzel, J., Andersen, H. and Daugaard Peters, N. (Section of Endocrinology, Department of Medicine, Department of Paediatrics and Department of Clinical Chemistry, Aalborg Regional Hospital, Aalborg, Denmark). Oxygen affinity of haemoglobin and red cell 2,3-diphosphoglycerate in childhood diabetes. Acta Paediatr Scand, 64:355, 1975.–Red cell 2,3-diphosphoglycerate (2,3-DPG) and the oxygen haemoglobin dissociation curve (ODC) were determined in 32 ambulatory, non-acidotic diabetic children and in 49 healthy children. Despite the fact that the diabetic children had, on average, an increased haemoglobin concentration, their erythrocytes contained significantly more 2,3-DPG than normal. Both in diabetic and in healthy children a negative relationship was found between the content of 2,3-DPG and the haemoglobin concentrations. No relationship was present between the plasma glucose and the 2,3-DPG concentration. The concentration of plasma inorganic phosphate (Pi) in the diabetic children was significantly higher than in the control children, and for all children there was a significant relationship between the 2,3-DPG and the Pi. In the diabetics 2,3-DPG was positively correlated to the P ₅₀ (7·40) and to the P ₅₀ (in vivo pH) of the ODC. However, despite the significant increase in 2,3-DPG among the diabetic children the average P ₅₀ (7·40) and P ₅₀ (in vivo pH) was not increased as compared with the control children. The inhibitory factor preventing the oxygen affinity from decreasing among the diabetics was strongly correlated to an increase in the mean corpuscular haemoglobin concentration. The result of this study suggests the presence of an increased amount of a haemoglobin fraction with high oxygen affinity (haemoglobin A_lc) in the red cells of juvenile diabetics.     

Red cell oxygen affinity, hemoglobin type, 2,3-diphosphoglycerate, and pH as a function of fetal development.

Studies were carried out on fresh cord blood obtained at delivery from nonstressed normal fetuses ranging from 24 to 42 weeks of gestation, to determine the relationship of 2,3-diphosphoglycerate (DPG), the intracellular red cell and extracellular pH, and the proportions of adult and fetal hemoglobin in regulating the position of fetal red cell oxygen affinity in utero. There was a significant positive correlation between P50 and gestational age (r = .62, P less than .001), the linear regression increased from 17.8 to 22.5 mm Hg. There was also a significant positive correlation between P50 and the percentage of adult type hemoglobin (HbA) (r = .67, P less than .001). In contrast gestational age had no effect of 2,3-DPG levels, the mean and SD was 14.86 +/- 2.04 mol/gm of Hb or delta pH between plasma and red cell, the mean was 0.187 +/- SD 0.032. However, there was a significant negative correlation between the intraerythrocyte hydrogen ion concentration and DPG level (r = .5, P less than .025). It is concluded therefore that the decrease in fetal oxygen affinity as gestation progresses is related mainly to the increase in the amount of HbA and the levels of DPG or delta pH between plasma and red cells are not a function of gestational age.     

The inhibition of the postnatal rise of 2,3-diphosphoglycerate in newborn lambs as a result of glucose perfusion

Due to the abrupt increase in 2,3-diphosphoglycerate (2,3-DPG) concentration in the newborn lamb, which begins soon after birth, this interval in development was considered an excellent period to test the hypothesis that glucose perfusions could inhibit 2,3-DPG synthesis. Ten newborn lambs were divided into two groups and perfused either with glucose (15 mg/kg/min) or physiologic saline (45% NaCl) for 10 days. Blood gases, electrolytes, glycemia, O2 pressure at 50% Hb saturation, and 2,3-DPG levels were compared in the two groups. Glucose levels remained significantly elevated during the first 3 days in the glucose perfused group. The O2 pressure at 50% Hb saturation increased in both groups but was significantly lower in the glucose perfused group when determined on day 5 and 8. The postnatal increase in 2,3-DPG was significantly diminished in the glucose infused lambs, which suggests that glucose perfusion has an inhibiting effect on erythrocyte 2,3-DPG synthesis.     

Increased erythrocyte Ca2+ content in hereditary spherocytosis.

The Ca2+ content of red blood cells (RBC) appears to correlate inversely with the Ca2+-ATPase activity of the red cell membrane (Fig. 1). When hereditary spherocytosis (HS) cells and normal control erythrocytes are compared, the dependence of Ca2+ content upon ATPase activity is similar. The Ca2+ content of HS red cells is increased compared with appropriate control erythrocytes matched for cell age (Fig. 2). Splenectomy has a different effect upon HS cells than upon normal cells (Fig. 2). Erythrocytes of splenectomized patients with HS seem to contain less Ca2+ than do those of presplenectomy patients with HS, whereas the reverse appears to pertain in normal subjects.     

Erythrocyte 2,3-diphosphoglycerate, PO250%, and available oxygen in young rabbits with and without postnatal fall in hemoglobin

We determined erythrocyte 2,3-diphosphoglycerate (2,3-DPG), PO250%, whole blood hemoglobin concentration, and available O2 from the 12th to the 30th d after birth in two matched groups of young rabbits. One group received iron parenterally on the 12th, 15th, and 18th d and the other received no iron supplement. In the untreated group there was a marked fall in hemoglobin concentration from the 12th to the 22nd d and thereafter a marked increase to the initial level on the 30th d whereas the iron-treated animals showed a marked rise in hemoglobin concentration from the 12th to the 22nd d, and a subsequent, slight decline from the 22nd to the 30th d. The average values of PO250% and 2,3-DPG, and the changes in PO250% and 2,3-DPG were virtually identical for both groups. During the first period (12-22 d) there was a marked rise in both 2,3-DPG and PO250% whereas in the second period (22-30 d) there was a somewhat smaller rise in 2,3-DPG and only a slight tendency toward a further rise in PO250%. In the untreated animals "available O2," reflecting the O2 delivery capacity of the blood, remained unchanged during the period of fall in hemoglobin concentration and showed a rise during the second period. In the iron-treated animals "available O2" rose markedly during the first period, with iron-treatment, and remained unchanged during the second period. We conclude that the marked postnatal rise in 2,3-DPG and PO250% in the rabbit seems to be independent of the changes in the hemoglobin concentration.     

Maternal cigarette smoking and fetal oxygen transport: a study of P50, 2,3-diphosphoglycerate, total hemoglobin, hematocrit, and type F hemoglobin in fetal blood

The current study was undertaken to determine whether the O2 carrying capacity of newborns born to mothers who smoke can accommodate to carbon monoxide poisoning to compensate for the potential tissue hypoxia induced by CO. From a cohort of 1,222 women and newborn infants the total hemogloblin concentration, hematocrit, and carboxyhemoglobin (HbCO) were measured and analyzed with regard to the maternal HbCO level and the number of cigarettes smoked per day. Also, for 100 pairs of mothers, of whom 50 were smokers and 50 nonsmokers, and their newborns, measurements were made of the intraerythrocyte concentration of 2,3-diphosphoglycerate (2,3-DPG), the hemoglobin concentration, and the affinity of hemoglobin for oxygen by determination of O2 tension for 50% of Hb-O2 saturation (P50). The results showed no significant relationship between smoking habits of mothers and their levels of hemoglobin, P50, and 2,3-DPG; only the hematocrit was slightly increased in smoking mothers. However, in fetal blood a significant correlation was found between the number of cigarettes smoked per day and levels of hemoglobin, hematocrit, and P50. The P50 decreased in relationship to the increase in maternal HbCO. The P50 change was brought about by an increase in hemoglobin F found in the fetuses of mothers who smoked. Although all these changes were statistically significant, these mechanisms of acclimatization to CO poisoning appeared trivial in magnitude. It is concluded that the human fetus does not have a biologic capacity to accommodate to maternal cigarette smoking, and therefore the fetus is particularly susceptible to the adverse effects of cigarette smoking.     

Changes in red cell phosphate metabolism of preterm and fullterm infants with perinatal problems during their first month of life

The effects of perinatal problems on red cell phosphate metabolism were studied in two groups of infants (preterms B and fullterms D) during the first month of life. All infants started milk feeding from day three after birth. The results were compared to those of healthy preterms (A) and fullterms (C), respectively. Comparisons were also made between the preterm and fullterm groups B and D. The preterms with perinatal problems (B) showed a significant delay in catching up with the plasma and red cell inorganic phosphate (Pi) levels of controls (A) throughout the first month of life (p < 0.05). In parallel, the erythrocyte 2,3 diphosphoglycerate (2,3-DPG) concentrations of the sick preterms lagged significantly behind those of controls (p < 0.001); but the ATP levels were comparable between the two groups. The fullterms behaved slightly differently. No significant differences in plasma Pi (Pl Pi) and red cell 2,3-DPG were seen between the sick and healthy neonates during the month of study, while red cell Pi (RBC Pi) and ATP were found to be lower in the sick ones (p < 0.05). The fullterms with perinatal problems (D) had significantly higher Pl Pi (p < 0.05) and RBC Pi (p < 0.01) than preterms with problems (B) from the first week of life and continued in a similar pattern until the end of the month. Red cell 2,3-DPG concentrations were found to be significantly correlated with Pl Pi and RBC Pi in both preterm groups (p < 0.01) and in the sick fullterms (p < 0.001) during the time of the study. In the healthy fullterms 2,3-DPG was found to correlate only with red cell Pi (p < 0.05). Perinatal problems seem to affect Pi metabolism to a different degree in preterm and fullterm neonates in the first month of life.     

Erythrocyte organic phosphates in the anemia of renal failure in childhood

Erythrocyte 2,3-diphosphoglycerate (2,3-DPG) and adenosinetriphosphate (ATP) levels were determined in 43 children with chronic renal failure on conservative treatment (CT), and 12 children on regular hemodialysis (HD) immediately before and after a HD session. The results were compared to non-anemic and anemic controls. In spite of anemia, erythrocyte 2,3-DPG in renal failure was similar to non-anemic controls at normal blood pH, but rose during dialysis as a result of alkalosis. In contrast, ATP levels were high already at a normal blood pH. 2,3-DPG correlated with packed cell volume (PCV) in children with renal failure but at lower concentrations compared to controls. Both organic phosphates in the erythrocytes showed a significant correlation with blood pH. The poor increase of 2,3-DPG, in combination with elevated ATP levels, suggests uremia-induced inhibition of 2,3-DPG synthesis.Dedicated to Prof. Dr. H. Bickel on the occasion of his 60th birthday     

Red cell phosphate metabolism in full-term neonates

The effects of hypoxia and feeding on red cell inorganic phosphate (Pi) concentrations were studied in neonates. Although hypoxia caused a rise in extracellular Pi, the intracellular concentration of this ion did not change in comparison to control infants of the same age (first 24 h). As a result of these changes, the distribution of phosphate ions across the erythrocyte membrane was significantly lower in the hypoxic infants than in the controls. In the hypoxic infants, adenosine triphosphate (ATP) levels in the red cells were found significantly lower than in controls, while the 2,3-diphosphoglycerate (2,3-DPG) levels were raised. In breast-fed 2 or 3-day-old neonates, both plasma and red cell Pi were found to be increased but to different degrees, affecting therefore the molar Pi distribution, which was lower than in the controls of the first day. In these infants, ATP was lower and 2,3-DPG higher than in the controls of the first day of life. These findings suggest that intracellular phosphate metabolism in neonates does not follow extracellular phosphate changes. Further investigation is needed to elucidate the controlling factors.     

Platelet dysfunction in asphyxiated newborn piglets resuscitated with 21% and 100% oxygen

Hemostatic disturbances are common in asphyxiated newborns after resuscitation. We compared platelet function in hypoxic newborn piglets reoxygenated with 21% or 100% oxygen. Piglets (1-3 d, 1.5-2.1 kg) were anesthetized and acutely instrumented for hemodynamic monitoring. After stabilization, normocapnic hypoxia was induced with an inspired oxygen concentration of 10-15% for 2 h. Piglets were then resuscitated for 1 h with 21% or 100% oxygen, followed by 3 h with 21% oxygen. Platelet counts and collagen (2, 5, and 10 microg/mL)-stimulated whole blood aggregation were studied before hypoxia and at 4 h of post-hypoxia/reoxygenation. Platelet function was studied using transmission electron microscopy and by measuring plasma thromboxane B2 (TxB2) and matrix metalloproteinase (MMP)-2 and -9 levels. Control piglets were sham-operated without hypoxia/reoxygenation. The hypoxemic (PaO2 33 mm Hg) piglets developed hypotension with metabolic acidosis (pH 7.02-7.05). Upon reoxygenation, piglets recovered and blood gases gradually normalized. At 4 h reoxygenation, platelet aggregation ex vivo was impaired as evidenced by a rightward-downward shifting of the concentration-response curves. Electron microscopy showed features of platelet activation. Plasma MMP-9 but not MMP-2 activity significantly increased. Resuscitation with 100% but not 21% oxygen increased plasma TxB2 levels. Platelet counts decreased after hypoxia/reoxygenation but were not different between groups during the experiment. Resuscitation of hypoxic newborn piglets caused platelet activation with significant deterioration of platelet aggregation ex vivo and increased plasma MMP-9 levels. High oxygen concentrations may aggravate the activation of prostaglandin-thromboxane mechanistic pathway.     

Postprandial gastrointestinal blood flow and oxygen consumption: effects of hypoxemia in neonatal piglets

The effects of feeding on gastrointestinal (GI) perfusion and oxygen transport in hypoxemic neonates is unknown. We evaluated these effects in unanesthetized, spontaneously breathing newborn piglets by comparing three experimental groups: nine hypoxemic piglets (mean PaO2 26 torr) which were fed with formula, six hypoxemic piglets (mean PaO2 27 torr) which were not fed, and four normoxemic piglets (mean PaO2 79 torr) which were fed and served as controls. The control-fed group exhibited an increase in stomach and small intestinal mucosal-submucosal blood flow within 30 min following feeding which was significantly greater than that observed in the hypoxemic fed piglets. GI O2 delivery and O2 uptake rose significantly (p less than 0.05) following a meal secondary to increases in total GI blood flow. Oxygen extraction was unchanged postprandially in the control group. In the hypoxemic nonfed piglets, total and regional GI blood flow was unaltered during hypoxemia. Reductions in arterial O2 content led to significant decreases in GI O2 delivery. Gastrointestinal oxygen uptake remained stable with a compensatory increase in GI O2 extraction. In the hypoxemic-fed piglets, hypoxia significantly decreased stomach blood flow and led to unchanged blood flow in the remainder of the GI tract. Significant reductions in arterial O2 content and GI O2 delivery were observed, accompanied by significant increases in O2 extraction. Hypoxemic fed animals did not exhibit the expected increase in O2 uptake to meet postprandial metabolic demands. When the hypoxemic insult was terminated, fed piglets demonstrated significant total and regional GI hyperemia leading to increased GI O2 uptake when compared with hypoxemic nonfed piglets.(ABSTRACT TRUNCATED AT 250 WORDS)     

Developmental changes in hemoglobin F levels during the first two years of life in normal and heterozygous beta-thalassemia infants

To study the developmental pattern of hemoglobin F (HbF) during the first two years of life, levels of HbF were estimated in two groups of infants: 117 normal infants and 98 heterozygotes for beta-thalassemia, all aged between 1 and 24 months. The results may be summarized as follows: (1) Levels of HbF in beta-thalassemia heterozygotes were significantly higher than those of normal infants of the same age (P less than .01). (2) A reference curve for the decline of HbF in infants with beta-thalassemia trait was established to facilitate the diagnosis of heterozygotes during this period of life. (3) Hemoglobin A2 (HbA2) was also higher in beta-thalassemia heterozygotes than in normal infants of the same age. HbA2 increases with increasing age, reaching normal adult values at age 5 to 6 months. It is postulated that the higher level of HbF in heterozygous infants during the first two years of life is associated with the presence of the beta-thalassemia gene, which influences the increased synthesis of HbF in red cell.     

HEMOGLOBIN-OXYGEN AFFINITY IN HYPOPHOSPHATEMIC RICKETS

Abstract. In 14 patients with simple X–linked hypophosphatemic rickets, 5 were below the third percentile in height and 9 were between the third and twenty-fifth percentile. Although the mean serum inorganic phosphorus level was only 2.01±0.65 (normal range for all age groups is 3.8 to 6.0 mg/100 ml), both the mean values for red cell 2,3-diphosphoglycerate (2,3-DPG) and adenosine triphosphate (ATP) were normal at 4.78±1.23 and 1.02±0.17 μmol/ml of red blood cells respectively. Moreover, the mean P₅₀ value was normal at 26.4±0.9 mmHg. These normal oxygen transport data make unlikely any proposal that short stature seen in these patients is secondary to chronic tissue hypoxia. They also indicate that the intra-erythrocytic organic phosphate levels are maintained at normal levels despite profound chronic hypophosphatemia.     

Hemoglobin-oxygen affinity in hypophosphatemic rickets.

In 14 patients with simple X-linked hypophosphatemic rickets, 5 were below the third percentile in height and 9 were between the third and twenty-fifth percentile. Although the mean serum inorganic phosphorus level was only 2.01 +/- 0.65 (normal range for all age groups is 3.8 to 6.0 mg/100 ml), both the mean values for red cell 2,3-diphosphoglycerate (2,3-DPG) and adenosine triphosphate (ATP) were normal at 4.78 +/- 1.23 and 1.02 +/- 0.17 mumol/ml of red blood cells respectively. Moreover, the mean P50 value was normal at 26.4 +/- 0.9 mmHg. These normal oxygen transport data make unlikely any proposal that short stature seen in these patients is secondary to chronic tissue hypoxia. They also indicate that the intra-erythrocytic organic phosphate levels are maintained at normal levels despite profound chronic hypophosphatemia.     

Red cell glycolytic intermediates and adenosine triphosphate in preterm infants on the first day of life

Red cell glycolytic intermediates and ATP were evaluated in 47 appropriate for gestational age preterm infants on the 1st day of life who were divided into three groups on the basis of gestational age: 28-30, 31-33, and 34-36 wk. The results were compared to those previously obtained in term infants. The concentrations of glucose-6-phosphate, total triose phosphates, and ATP were significantly higher than in term infants but appeared to be appropriately elevated for the young mean age of the red cell population. The concentration of red cell 2,3-diphosphoglycerate (2,3-DPG) was significantly decreased when compared to term infants and was lowest at 28-30 wk gestation. The content of red cell 3-phosphoglycerate was increased in term infants and was inappropriately elevated for the age of the red cell population at 28-30 wk gestation. This pattern of glycolytic intermediates was suggestive of a young red cell population metabolizing at an increased glycolytic rate with increased flow through the phosphoglycerate kinase step rather than the 2,3-DPG bypass in "normal" preterm infants. Two preterm infants of 28-30 wk gestation with low red cell intracellular pH were also evaluated and had markedly decreased concentrations of red cell 2,3-DPG and ATP and all phosphorylated intermediates distal to the phosphofructokinase reaction, indicative of a cross-over at the phosphofructokinase step secondary to acidosis.(ABSTRACT TRUNCATED AT 250 WORDS)     

2,3-DIPHOSPHOGLYCERATE LEVELS IN CHILDREN WITH IRON DEFICIENCY ANEMIA AND ACUTE LEUKEMIA

Abstract. Red cell organic phosphates and especially 2,3-diphosphoglycerate (2,3-DPG), lowers the oxygen affinity of hemoglobin (Hb) and shifts the oxygen dissociation curve to the right. Because of the importance of 2,3-DPG (as regulator of the oxygen affinity of Hb), determinations were carried out on: 45 normal children, 7 children with iron deficiency anemia and 35 children with acute lymphoblastic leukemia. In normal children with Hb of 12.69±1.60 g/100 ml, 2,3-DPG was 14.90±0.68 μmoles/g Hb. In children with iron deficiency anemia (Hb 7.94±1.20 g/100 ml), 2,3-DPG was 20.87±3.11 μmoles/g Hb. 2,3-DPG was normal (14.11±0.88 μmoles/g Hb), in 16 patients with acute lymphoblastic leukemia who had never relapsed, while in 19 patients with a history of one or more relapses, 2,3-DPG levels were increased (22.05 ±2.75 μmoles/g Hb). No good explanation may be offered for the high 2,3-DPG levels in these leukemic children.     

Changes of blood oxygen affinity and capacity and red cell 2,3-diphosphoglycerate evoked by exchange transfusions with ACD preserved blood in newborn infants: Their interrelationship and influences on oxygen supply of tissues and erythropoiesis

In 20 newborns and infants suffering from hyperbilirubinemia due to blood group incompatibility and other causes in vivo blood oxygen affinity and capacity, acid-base equilibrium, reticulocyte count, and red cell 2,3-diphosphoglycerate (2,3-DPG) were determined before and after exchange transfusion (ET) with ACD preserved blood. Functional and biochemical revival of donor blood occurs in vivo during ET; in vitro revival of comparable extent is observed after THAM buffering of the ACD stored blood prior to ET. Normal properties of adult blood in the recipient infants are reached after 24 hrs. A further increase of 2,3-DPG with the resulting enhanced oxygen releasing ability of hemoglobin occurs within 3 to 6 days. Lateron 2,3-DPG and the oxygen releasing ability are being reduced to subnormal values. The resulting decrease of mixed venous oxygen tension at this time is followed by reticulocytosis. Thus 2,3-DPG can be assumed as a regulator of erythropoiesis. Since our results demonstrate a possibility to manipulate the oxygen carrying characteristics of blood of the newborn, factors which are to be considered in states of severe shunt hypoxemia are outlined.Presented in part at XIII. Internat. Congr. Pediatrics, Vienna 1971, and at the Internat. Symposium on Synthesis, Structure and Function of Hemoglobin, Bad Nauheim, April 1971.     

Chronic hypoxemia in the newborn lamb: cardiovascular, hematopoietic, and growth adaptations

We have created a model of chronic hypoxemia in the newborn lamb by decreasing pulmonary blood flow in the presence of an atrial septal defect. Via a left lateral thoracotomy, we place an inflatable balloon around the pulmonary artery and perform an atrial septostomy under direct vision. We also insert several vascular catheters and place an electromagnetic flow transducer around the ascending aorta. Three days after surgery, we inflated the balloon in 11 lambs such that arterial oxygen saturation decreased to 60 to 75%. Studies were performed on these lambs twice weekly and weekly on 12 normoxemic lambs. Growth decreased sharply (47 +/- 123 versus 221 +/- 82 g/day) at the onset of hypoxemia and remained low, although oxygen consumption followed the normal gradual decline. Heart rate remained elevated throughout the study. Arterial PCO2 levels decreased from 40 +/- 5 to 35 +/- 7 torr and remained low. Systemic blood flow decreased at balloon inflation but quickly returned to normal. Mixed venous saturation was low, but could decrease further with shivering. Systemic oxygen delivery decreased initially but returned to normal as Hb concentration rose (from 9.4 +/- 1.5 to 12.5 +/- 2.2 g/dl). P50 increased normally over the study period. Four of the 11 hypoxemic lambs died during the study. These data show that, in the chronically hypoxemic newborn, systemic oxygen delivery is maintained primarily by a rising Hb. Total body oxygen consumption is maintained at rest but is redistributed away from anabolic requirements and toward cardiorespiratory work. This signal to decrease growth occurs despite less than maximal oxygen extraction at rest.     

Hematocrit correlates well with circulating red blood cell volume in very low birth weight infants

Although circulating red blood cell (RBC) volume is a better measure of total body oxygen delivering capacity than hematocrit (HCT), circulating RBC volume is more difficult to measure. Thus, the HCT is often used in RBC transfusion decisions. However, several previous studies of low birth weight infants have reported that the correlation between HCT and circulating RBC volume is poor. Using a robust nonradioactive method based on in vivo dilution of biotinylated RBC enumerated by flow cytometry, the present study reexamined the correlation between HCT and circulating RBC volume in very low birth weight infants. Venous and capillary HCT levels were compared with circulating RBC volume measured using the biotin method. Twenty-six stable very low birth weight infants with birth weights less than 1300 g were studied on 43 occasions between 7 and 79 d of life. Venous HCT values correlated highly with circulating RBC volume (r = 0.907; p < 0.0001). However, the mean 95% confidence limits for prediction of circulating RBC volume from venous HCT (the average error of prediction) was +/-13.4 mL/kg. The correlation between HCT and circulating RBC volume is strong in older stable very low birth weight infants. However, clinically important uncertainty exists in estimating circulating RBC volume and the associated RBC transfusion needs of an individual infant based on venous HCT. Because direct measurement of circulating RBC volume is not yet practical, the HCT (or the blood Hb concentration) remains the best available indirect indicator.