Use of the retinol-binding protein: transthyretin ratio for assessment of vitamin A status during the acute-phase response

The ratio plasma retinol-binding protein (RBP):transthyretin (TTR) has been proposed as a means to improve the assessment of vitamin A status of indiv iduals with concurrent infection or inflammation. We have measured RBP and TTR in stored sera from South African children who had accidentally ingeste d kerosene. Samples were collected from these children in hospital when suf fering acute inflammation and respiratory distress, and from them and neigh bourhood control children 3 months later. Vitamin A status was defined by m odified relative dose response (MRDR) tests of liver retinol stores at 3 mo nths and by serum retinol concentration both when children were ill and whe n they were well. Illness was defined as either being in hospital or, at fo llow-up, as having a raised plasma alpha(1)-acid glycoprotein (AGP) level. The RBP:TTR value was significantly decreased by both illness and low liver retinol stores. When the effects on RBP:TTR of illness and vitamin A store s were considered together for the 3-month follow-up samples, only vitamin A status significantly decreased the value. We calculated sensitivity and s pecificity of the RBP :TTR ratio against established measures of vitamin A status using a Cut-off value of 0.3 for RBP:TTR and standard cut-off values for MRDR (0.06) and plasma retinol (0.7 mu mol/l). Compared with MRDR, RBP :TTR had sensitivities of 76 % and 43 % and specificities of 22 % and 81 % to detect vitamin A deficiency in hospitalized and well children respective ly. Compared with plasma retinol, sensitivities were 88 % and 44 % and spec ificities were 55 % and 64 % in hospitalized and well children respectively . Only for the case of clinically well children with biochemical evidence o f subclinical inflammation did sensitivity (62 % and 100 % against MRDR and plasma retinol respectively) and specificity (100 % and 60 % against MRDR and retinol) approach useful levels for an assessment tool. Overall, althou gh a trend supporting the theory behind the use of the RBP:TTR for assessme nt of vitamin A status in infection was observed in the current study, the ratio did not provide adequate sensitivity and specificity to be a useful a ssessment tool.