Patient Self-Testing of Kidney Function at Home, a Prospective Clinical Feasibility Study in Kidney Transplant Recipients

Abstract

Introduction: People with long-term health conditions often attend clinics for kidney function tests. The Self-Testing Own Kidneys (STOK) study assessed feasibility of kidney transplant recipients using hand-held devices to self-test kidney function at home and investigated agreement between home self-test and standard clinic test results.

Methods: A prospective, observational, single-center, clinical feasibility study (TRN: ISRCTN68116915), with N = 15 stable kidney transplant recipients, investigated blood potassium and creatinine results agreement between index self-tests at home (patient self-testing of capillary blood, using Abbott i-STAT Alinity analyzers [i-STAT]) and reference tests in clinic (staff sampled venous blood, analyzed with laboratory Siemens Advia Chemistry XPT analyzer) using Bland-Altman and error grid analysis.

Results: The mean within-patient difference between index and reference test in creatinine was 2.25 μmol/l (95% confidence interval [CI]: -12.13, 16.81 μmol/l) and in potassium was 0.66 mmol/l (95% CI: -1.47, 2.79 mmol/l). All creatinine pairs and 27 of 40 (67.5%) potassium pairs were judged clinically equivalent. Planned follow-up analysis suggests that biochemical variables associated with potassium measurement in capillary blood were predominant sources of paired test result differences. Paired patient and nurse i-STAT capillary blood test potassium results were not statistically significantly different.

Conclusions: This small feasibility study observed that training selected patients to competently use hand-held devices to self-test kidney function at home is possible. Self-test creatinine results showed good analytical and clinical agreement with standard clinic test results. Self-test potassium results showed poorer agreement with standard clinic test results; however, patient self-use of i-STATs at home was not a statistically significant source of difference between paired potassium test results.

Keywords: creatinine; home; kidney; point-of-care test; potassium; self-test.

Graphical abstract

Figure 1

Pathway completed by Self-Testing Own Kidneys (STOK) Study participants, comprising study consent, training, home and clinic testing, and evaluation. CRF, Case Report Form.

Figure 2

Bland-Altman plot of creatinine (unit = μmol/l) showing the difference for each patient, at each time point, between the i-STAT at home (capillary blood) and standard laboratory testing (venous blood), plotted against the mean of the 2 tests. The dashed reference line (y = 0) represents zero difference between tests (i.e., perfect agreement), the 2 dot-dash lines (y = 16.81, −12.31) represent the 95% confidence interval for differences between pairs of individual measurements, and the 2 solid lines (y = 25, y = −25) represent the clinical acceptability criteria. Participant ID has been used as the data-point symbol. Green symbols represent clinically acceptable differences between measures (i.e., data points located in-between the 2 solid lines), whereas red symbols represent clinically unacceptable differences (i.e., data points located outside the 2 solid lines); there are no clinically unacceptable paired results for creatinine.

Figure 3

Bland-Altman plot of potassium (unit = mmol/l) showing the difference for each patient, at each time point, between the test result for the i-STAT at home on capillary blood and standard laboratory testing on venous blood (Pat_iSTAT – Lab), plotted against the mean of the 2 tests— (Pat_iSTAT + Lab)/2. The dashed reference line (y = 0) represents zero difference between tests (i.e., perfect agreement), and the 2 dot-dash lines (y = 2.79, y = −1.47) represent the 95% confidence interval for differences between pairs of individual measurements. Participant ID has been used as the data-point symbol. Green symbols represent clinically acceptable differences between paired results, whereas red symbols represent clinically unacceptable differences.

Figure 4

Error grid plot of potassium (unit = mmol/l) showing the result for each patient, at each time point, of standard laboratory testing on venous blood (Lab) plotted against the test result using the i-STAT at home on capillary blood (Pat_i-STAT). The diagonal dashed reference line represents zero difference between tests (i.e., perfect agreement). The data points inside area A, which comprises the rectangular area and the 2 adjoining corridors, represent the clinically acceptable paired results. The data points in area B, which comprises the entire area outside of area A, represent the clinically unacceptable paired results. Participant ID has been used as the data-point symbol. Green symbols represent clinically acceptable differences between paired results, whereas red symbols represent clinically unacceptable differences.

Figure 5

Box plots showing the comparisons in the linear mixed effect modeling, as part of the planned framework of follow-up analyses on potassium (unit = mmol/l). The lower and upper hinges of the boxplots correspond to the first and third quartiles (i.e., the 25th and 75th percentiles). The upper and lower whiskers extend from the hinge to the largest and smallest value, respectively. The bolded, horizontal line inside the boxplot represents the median value. Outliers were determined using the 1.5 × IQR rule (Q₁ – 1.5 × IQR, Q3 + 1.5 × IQR) No outliers were detected. Plot A shows the comparison between the patient use of the i-STAT at home and the clinic use of the i-STAT, both on capillary blood.