Haemoglobin‐S in sickle cell trait with papillary necrosis

Sickle cell trait (SCT) is the carrier state of sickle cell anaemia (Flint et al, 1993; Davies & Oni, 1997). However, in SCT, only 20–40% of the total haemoglobin is Hb S, and the abundance of normal Hb A (60–80%) prevents sickling under most physiological circumstances (Barbedo & McCurdy, 1974). Furthermore, the level of Hb S is lower when SCT co-exists with a-thalassaemia (Barbedo & McCurdy, 1974). Nonetheless, SCT red cells do occasionally sickle under the hyper-osmolar conditions encountered in the renal papillae, resulting in renal papillae necrosis (RPN) and haematuria (Barbedo & McCurdy, 1974). This study aimed to identify any association between Hb S levels and the development of RPN by determining the HbS levels of SCT individuals in Maiduguri, Nigeria. A total of 52 subjects with SCT who had haematuria because of RPN diagnosed by renal ultrasonography between 1996 and 2004 at the University of Maiduguri Teaching Hospital, Nigeria, were evaluated with respect to leucocyte count, platelet count, total haemoglobin concentration, Hb S levels, mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH) and mean corpuscular haemoglobin concentration (MCHC). All parameters were determined by Beckman Coulter AcT-8 except for Hb S, which was determined by a manual elution technique (Dacie et al, 1991) and expressed as the percentage of total haemoglobin. Equal numbers of ageand sex-matched individuals with SCT who had no history of haematuria and had no ultrasonographic features of RPN were also recruited as controls and evaluated with respect to similar parameters in a case–control study. All the subjects included in this study were confirmed cases of SCT, established by a positive sickling test and Hb AS electrophoretic pattern (Dacie et al, 1991). Student t-test was used to compare the mean values of the studied parameters between the two groups; a probability level of P < 0Æ05 was taken as significant. Conditional logistic regression using the Statistical Package for the Social Sciences (spss) software, version 11.0 (SPSS Inc., Chicago, IL, USA), was performed to assess the risk of RPN because of Hb S levels among the subjects studied. The results of this study are shown in Table I. All subjects had normal haematological parameters. The SCT subjects with a history of RPN did not significantly differ from the control group with respect to mean age, sex ratio, leucocyte and platelet count. The red cell indices, MCV, MCH and MCHC, were normal and did not significantly differ between the two groups of SCT subjects studied (P > 0Æ05). However, SCT subjects with a history of RPN had a significantly higher mean Hb S level (34%) than that seen in control subjects (25%) (P < 0Æ05). Conditional logistic regression for the two ageand sex-matched groups with respect to the risk of RPN because of Hb S levels gave an odds ratio (OR) of 1Æ58 (95% confidence interval: 0Æ33–6Æ61) with a significant P-value of 0Æ03. The finding of normal total haemoglobin concentrations among the SCT subjects studied in this report confirmed the absence of haemolysis in persons with SCT, which was consistent with earlier reports (Beutler, 1991). Furthermore, the finding of normal values of red cell indices among our SCT subjects would strongly suggest the absence of a-thalassaemia (Dacie et al, 1991). This study further revealed that SCT subjects with a history of RPN had a higher mean Hb S level, indicating that RPN was associated with higher levels of Hb S in SCT. This result would suggest that high Hb S level is a risk factor for the development of RPN in SCT subjects. The significant OR (1Æ58) would suggest a higher relative risk as the level of Hb S increases. However, it should be noted that the actual magnitude of the relative risk would be best determined by a cohort study. The present study found that SCT subjects with a history of RPN were identifiable as a