Page 197 - HIVMED_v21_i1.indb

Page 3 of 15 Original Research

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the stability of biochemical analytes is 10. Furthermore, Estimated glomerular filtration rate
multiple samples were required from each participant measurements
for analysis on three analysers at six time points, providing eGFR was calculated using the MDRDv3 without ethnic
396 creatinine measurement data points. Patients with
comorbidities such as diabetes mellitus, pregnancy, urinary adjustment (age, sex, and serum creatinine), MDRDv4 (age,
tract infection, hypertension and proteinuria were excluded sex, ethnicity, and serum creatinine), CG and CKD-EPI
from the study. De-identified demographic and anthropometric equations with ethnic adjustment.
data were transcribed from the patient files.
Statistical analysis
Sample collection and processing All analyses were performed using Medcalc software. The
Each patient provided 13 tubes of blood: 12 × approximately Shapiro-Wilks test was used to test for normality. Normally
2 mL in 5 mL serum separator-tubes (SST) and 1 × 5 mL in a distributed continuous variables (creatinine) were presented
heparin tube (Becton Dickenson, Plymouth, UK). Serum was as mean ± standard deviation and continuous variables that
isolated from SST tubes following centrifugation at 1370 × g were not normally distributed (age, weight) were presented
for 5 min at defined time intervals viz. < 4 h, 6 h, 24 h, 48 h, as median [interquartile range]. Categorical variables are
72 h and 96 h. Prior to centrifugation, samples were stored at presented as proportions and percentages. The Student
room temperature. Whole blood heparin tubes were stored at paired t-test was used to compare baseline creatinine
room temperature until required. concentrations between the three different methods
(Bonferroni corrected p-value). The serum creatinine results
Measurement of creatinine concentrations were evaluated for analytically significant changes using the
uncertainty of measurement (UOM; calculated for the
Serum creatinine concentrations were measured immediately laboratory quality control data in the preceding 6 months),
after each centrifugation (as above) using the IDMS-traceable total allowable error (TAE) of 15% (Clinical Laboratory
enzymatic and IDMS-traceable kinetic Jaffe method on the Improvement Amendments [CLIA] and coefficient of
Roche COBAS 8000 module 702 and Roche COBAS 6000 variation [CV]). Passing-Bablok linear regression and Bland-
module 502 instruments at the NHLS laboratory of the Chris
Hani Baragwanath Hospital, Soweto, SA. The enzymatic Altman plots were generated to evaluate correlation and
method agreement over time, using the enzymatic method as
method test principle is based on a series of coupled reactions
that result in the conversion of creatinine to hydrogen the reference method. For all analyses, p < 0.05 was considered
peroxide. In the final reaction, catalysed by peroxidase, a statistically significant.
quinone imine chromogen is formed. The colour intensity of
this chromogen is measured at 550 nm and is directly Ethical consideration
proportional to the concentration of creatinine in the sample. Ethical approval was obtained from the University of the
The kinetic Jaffe method test principle is based on the reaction Witwatersrand Human Research Ethic Committee, reference
of creatinine and picrate under alkaline conditions forming a number: M1711109. R14/49.
yellow-orange complex. The rate of formation of this complex
is proportional to the amount of creatinine in the sample. Results
The kinetic Jaffe method is prone to interference from non-
creatinine chromogens such as proteins and ketones. Characteristics of the study population
A correction factor of -26 µmol/L is, therefore, universally A total of 22 randomly recruited HIV-positive, treatment-
applied to correct for these inherent chromogens. Bilirubin is naïve black South African individuals consented to
the major negative interferent in the kinetic Jaffe method, and participate in this study. The median age of the participants
this is overcome by rate blanking. The internal quality control was 37 years. The majority (54.5%; 12 of 22) of participants
for creatinine in our laboratory is performed twice daily. The were younger than 40 years of age. The age distribution is
laboratory adheres to the Royal College of Pathologists of consistent with the current demographics published by
Australasia (RCPA) quality assurance programmes. External Statistics South Africa with respect to individuals mostly
quality assurance submissions during the study period were affected by HIV/AIDS. All participants were black Africans
within allowable limits. (ethnicity self-reported). The median weight for the
participants was 70.0 kg (range 45–110 kg) and 59.1% (13 of
Creatinine concentrations were also measured using whole 22) of participants were male.
blood samples on the Abbott i-STAT point-of-care instrument
immediately after collection and at 6 h, 24 h, 48 h, 72 h and
96 h. This method is based on the conversion of creatinine Creatinine assay performance
to hydrogen peroxide, via hydrolysis and oxidation. The mean baseline (< 4 h) creatinine concentrations for the
The hydrogen peroxide is oxidized (at the platinum electrode) study cohort obtained using the enzymatic, kinetic
to produce a current that is proportional to the concentration Jaffe method and i-STAT methods were 78.73 µmol/L
of creatinine in the sample. ±14.63 µmol/L, 77.05 µmol/L ± 13.12 µmol/L and

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