Abstract
A serum calcium-phosphorus (sCaPP) product was assessed for prediction of survival in dogs affected with chronic kidney disease (CKD). Dogs (N = 150) were retrospectively studied and followed up to determine their lifespan using 25 healthy dogs as controls. Blood and urine analyses were performed and blood pressure was measured. The dogs were divided into groups according to sCaPP (higher or lower than 70 mg2/dL2) and International Renal Interest Society (IRIS) stage (IRIS 1–4). Shorter survival was observed with sCaPP > 70 mg2/dL2 compared to dogs with sCaPP < 70 mg2/dL2 [45.48 days (range: 5.8 to 149 days) versus 505.40 days (range: 113.31 to 539.52 days), mean (95% confidence interval); P ≤ 0.001 respectively]. Similarly, dogs with advanced IRIS stages showed higher levels of sCaPP [mean (95% confidence interval) in mg2/dL2; IRIS 1: 42.83 (range: 29.58 to 62.10); IRIS 2: 63.18 (range: 46.34 to 90.09); IRIS 3: 95.57 (range: 88.34 to 127.19); IRIS 4: 130.38 (range: 125.16 to 153.52)], accompanied by lower survival rates. Therefore, sCaPP could represent a valuable tool in the prognosis of canine CKD.
Résumé
Un produit plasmatique calcium-phosphore peut être utilisé pour prédire la durée de vie de chiens avec une maladie rénale chronique. Un produit sérique calcium-phosphore (sCaPP) fut évalué pour prédire la survie de chiens souffrant de maladie rénale chronique (CKD). Des chiens (N = 150) furent étudiés rétrospectivement et suivis pour déterminer leur survie en utilisant 25 chiens en santé comme témoins. Des analyses urinaires et sanguines furent effectuées et la pression sanguine fut mesurée. Les chiens furent divisés en groupes en fonction de leur sCaPP (plus élevé ou plus faible que 70 mg2/dL2) et de leurs stages selon l’International Renal Interest Society (IRIS) (IRIS 1–4). Un temps de survie plus court fut observé avec une sCaPP > 70 mg2/dL2 comparativement aux chiens avec une sCaPP < 70 mg2/dL2 [45,48 jours (varie de 5,8 à 149 jours) versus 505,40 jours (varie de 113,31 à 539,52 jours), moyenne (intervalle de confiance 95 %); P ≤ 0,001 respectivement]. De manière similaire, les chiens avec un stages IRIS avancé avaient des niveaux de sCaPP plus élevés [moyenne (intervalle de confiance 95 %) en mg2/dL2; IRIS 1 : 42,83 (varie de 29,58 à 62,10); IRIS 2 : 63,18 (varie de 46,34 à 90,09); IRIS 3 : 95,57 (varie de 88,34 à 127,19); IRIS 4 : 130,38 (varie de 125,16 à 153,52], accompagnés de taux de survie plus bas. Ainsi, la valeur de sCaPP pourrait représenter un outil utile dans le pronostic des maladies rénales chroniques chez le chien.
(Traduit par Dr Serge Messier)
Introduction
Chronic kidney disease (CKD) is defined as the presence of structural and functional abnormalities in one or both kidneys over an extended period of time exceeding 3 mo (1). Chronic kidney disease is caused by numerous and heterogeneous etiological factors (familial, congenital, or acquired) that cause overall reduction of the glomerular filtration rate (GFR) resulting in dramatic consequences on the patient’s homeostasis (2). Estimates of the etiologic prevalence of CKD vary widely depending on the selected canine population and the inclusion criteria, ranging from 0.05% to 3.74% (3–7).
The progressive reduction of functional nephrons directly influences the solutes excreted in the urine, including phosphorus (8). Hyperphosphatemia is a triggering factor in the development of renal secondary hyperparathyroidism (RSHP), which is considered a common complication, with increased parathyroid hormone (PTH) levels in 76% of the affected dogs (5,9). Serum phosphorus concentrations have been shown to be directly related to increased mortality in humans, cats, and dogs (4,10,11). Maintenance of normal serum phosphate levels is regulated by a complex system involving bones, intestines, kidneys, and the parathyroid gland. The main players in the regulation of phosphate are: PTH, calcitriol, and fibroblast growth factor 23 (FGF23), a phosphaturic glycoprotein. Elevated levels of serum phosphate increase the secretion of PTH and FGF23, both of which inhibit phosphate reabsorption in the kidney promoting phosphaturia, while FGF23 decreases phosphate gut reabsorption (for a comprehensive review see reference 12). It is known that phosphorus retention is associated with the development and progression of RSHP (4,13,14). Among other damaging effects, hyperphosphatemia exacerbates the progression of CKD by precipitating calcium in the renal interstitium in the form of calcium phosphate, resulting in interstitial fibrosis and tubular atrophy (4).
Although hyperphosphatemia is accepted as a frequent finding in dogs with CKD (15–18), there are few studies that correlate its increase with the severity of the disease. In 2009, Cortadellas et al (15) showed that hyperphosphatemia (phosphorus concentration > 1.78 mmol/L) was present in 12%, 50%, 76.9%, and 100% of dogs in stages 1, 2, 3, and 4 of the IRIS classifications, respectively. Gerber et al (16), reported that RSHP was present in 100% of dogs with moderate or severe CKD, whereas the concentration of calcitriol was significantly lower compared to a control group, but still within the reference range in most dogs.
Serum total calcium concentration (tCa) is composed of 3 fractions: ionized, protein-bound, and complexed, with each comprising approximately 56%, 34%, and 10% of the serum tCa, respectively. Ionized calcium concentration (iCa) is the most biologically active fraction and also is a sensitive indicator of pathologic states when abnormal, but its concentration has to be measured using specific analyzers not always available in veterinary hospitals. Furthermore, iCa can vary due to pH, lactate, protein concentration, and does not reliably correlate with total serum calcium, largely varying in dogs affected with CKD in which hypoproteinemia is a common finding (19,20). To improve the accuracy of tCa to predict iCa, correction equations involving total protein or albumin concentrations are used, although their use does not yield reliable values (19).
The combination of hyperphosphatemia and a normal calcium concentration in plasma results in a high serum calcium-phosphorus concentration product (sCaPP). If it exceeds ~ 70 mg2/dL2 there is a tendency for calcium phosphate to precipitate in blood vessels, joints, and soft tissues as described in humans (21). This process is known as metastatic calcification and is especially important in organs that secrete protons, such as the stomach and kidneys, although other organs and tissues such as the myocardium, lung, and liver are commonly mineralized in patients with CKD (4).
In humans, elevated sCaPP is associated with poor prognosis and high risk of vascular calcification (10,22), myocardial infarction, and coronary calcification (23–25).
Cortadellas et al (9) established a correlation between sCaPP and PTH concentration in dogs affected with CKD. In 2014, Lippi et al (21), in a study using 31 dogs diagnosed with CKD, demonstrated that a sCaPP > 70 mg2/dL2 is correlated to poorer CKD prognosis and short-term mortality (mean survival = 30 d) regardless of the plasma creatinine concentration.
Accordingly, sCaPP is widely used as an indicator of survival in CKD in human medicine, although it has not been sufficiently studied in large populations of dogs with naturally occurring CKD. Thus, our aim was to assess whether sCaPP can be used to predict lifespan in a large population of dogs affected with CKD using the reference value previously established as a threshold (25).
Materials and methods
Animals
A retrospective study was carried out in 150 dogs affected with CKD diagnosed at the Small Animal Internal Medicine Service of the Veterinary Clinical Hospital (VCH) of the University of Extremadura (UEx, Spain) from January 2014 to January 2017. The mean age of the 79 male and 71 female dogs diagnosed with CKD was 7.24 y (range: 6.47 to 8.23 y).
Twenty-five healthy dogs attending the VCH of the UEx, for periodic examinations or for sterilization were used as controls. The median age of the group of healthy dogs used was 2.75 y (range: 1 to 9 y), including 12 males and 13 females. To confirm their health status, a medical history, complete physical examination, systolic blood pressure determination, complete blood (cell) count (CBC) (hematology and biochemical profile), and urinalysis were performed.
In all cases, the complete clinical history and the treatment prescribed were carefully reviewed; the parameters recorded were the same as for the healthy dogs. The patients in this group were presented with renal azotemia (defined as a plasma creatinine concentration > 115 μmol/L and urea > 9.16 mmol/L), according to the reference values of the Laboratory of Clinical Pathology of VCH of the UEx. Dogs were also classified according to the guidelines established by the International Renal Interest Society (IRIS).
Renal azotemia was persistent after the correction of prerenal factors (if present) being maintained or even worsening during the study. The patients were periodically reviewed depending upon the severity of their CKD and the data were acquired and registered at each revision. Dogs staged as IRIS 1 and 2 were checked every 3 to 6 mo, dogs classified as IRIS 3 were evaluated every 2 to 3 mo and weekly follow-ups were scheduled for the IRIS 4 patients. At each follow-up, a complete physical examination, CBC, biochemistry, and urinalysis were performed to confirm CKD progression.
Dogs with urinary tract infections were excluded. Results of the blood and urine tests used herein correspond to the day in which CKD was diagnosed (day 1), before the onset of the treatment.
This study was not subjected to animal ethics committee review as all the animals enrolled in this study were dogs referred to the VCH of the UEx. The control group samples were obtained from dogs referred for castration or regular health check and only the excess of blood and urine samples were used for this study with consent from the owner.
Systolic blood pressure determination
Systolic blood pressure was determined using a non-invasive Doppler detector (Parks Medical Model 811 Ultrasonic Doppler 8.9 MHz Flow Detector; Aloha, Oregon, USA). The transducer was placed over the proximal carpal artery, digital palmar artery, saphenous branch, or medial plantar artery. A cuff was placed according to the patient’s limb size and successive measurements were taken until a stable value was obtained. Blood pressure over 160 mmHg was considered as abnormal (hypertension).
Blood and urine tests
Hematological, biochemical, and urinary tests were performed on each dog aiming to stage the severity of their CKD (IRIS 1 to 4). Hematology was performed on blood samples extracted using K3-EDTA tubes and the samples were analyzed using an automatic hematology analyzer (Mindray BC-5300; Shenzhen Mindray Bio-Medical Electronics, Chinese Mainland, People’s Republic of China). The erythrocyte count, hematocrit value, mean corpuscular volume (MCV), mean corpuscular hemoglobin concentration (MCHC), total and differential leukocyte count, and platelet count were determined.
Blood biochemistry was performed in plasma using a Saturno 100 analyzer; (Crony Instruments, Rome, Italy). Plasma was obtained after centrifugation of a separate blood sample extracted with sodium heparin (2000 × g for 10 min at room temperature). The parameters analyzed were urea, creatinine, calcium, phosphorus, total proteins, albumin, sodium, and potassium (Spinreact reagents; Spinreact, Gerona, Spain). Calcium concentration was corrected as a function of plasma albumin concentration:
Corrected calcium (mmol/L) = measured calcium (mmol/L) — albumin (g/L) + 3.5. The sCaPP was calculated by multiplying serum total calcium concentration by serum phosphorus concentration.
Urine was analyzed from samples obtained by an ultrasound-guided cystocentesis. A qualitative and semi-quantitative examination included a urinary strip (Multistix; Bayer Corporation, Valencia, Spain) and analysis of the urine sediment by microscopy, after centrifugation of the urine for 5 min at 200 × g. Urinary density was measured by refractometry (two-scale clinical refractometer; Zuzi, Beriain, Spain). Protein concentration (pirogallol red, RAL; laboratory, automatic analyzer Saturno 100; Crony Instruments) and creatinine concentration (Jaffé reaction; RAL Laboratory, Barcelona, Spain, for Saturno 100 automatic analyzer) were also determined. The protein/creatinine ratio (UP/C) was calculated to determine the presence of proteinuria.
All dogs were tested for the presence of antibodies for Ehrlichia canis, canine dirofilariosis, Borrelia burgdorferi, and Anaplasma phagocytophylum using a SNAP test (Canine SNAP 4Dx Plus; IDEXX Laboratories, Hoofddorp, The Netherlands) following the routine protocol of the VCH for CKD diagnosis. The diagnosis of leishmaniasis was carried out by direct visualization of amastigotes in lymph node or bone marrow aspirates and/or an enzyme-linked immunosorbent assay (ELISA) using a Leti Q letitest ELISA kit (Laboratorios Leti, Madrid, Spain).
Survival records
For the control group, survival was assessed for at least 6 mo. The survival of each patient was estimated from the day at which CKD was diagnosed (day 1) to the day of death. Only those animals with a complete clinical follow-up were considered. The duration of the study varied for each patient and was extended for 3 y in some patients. All dogs allocated to the IRIS 1 group were followed for a minimum of 2 y.
In all cases, a specific diet was prescribed for renal disease. Dogs staged at IRIS 2, 3, or 4 disease received calcium carbonate as a phosphate binder. Angiotensin-converting enzyme inhibitors as antihypertensive agents were prescribed in dogs with UP/C > 0.5 and/or with systolic blood pressure >160 mmHg. Other treatments (e.g., antacids and antiemetics) or specific treatments for erhlichiosis (doxycycline) and leishmaniasis (miltefosine or allopurinol) were initiated depending upon the patient’s clinical signs. Calcitriol was not given to any of the enrolled animals.
Statistical analysis
Statistical analysis was performed with the SPSS v19.0 Statistical Data Analysis software (IBM, Armonk, New York, USA). The data were first analyzed using a Shapiro-Wilk test to assess the normality of the data distribution; normally distributed data were total protein, hematocrit, albumin, arterial pressure, and MCV. The remaining parameters measured presented a non-Gaussian distribution (erythrocyte count, MCHC, leukocyte count, platelet count, sodium, urine density, and survival).
To establish differences between values among groups, data were compared using a Kruskal-Wallis H-test for non-normally distributed data and a 1-way analysis of variance (ANOVA) followed by a Bonferroni post-hoc test for normally distributed parameters. Differences between groups were considered statistically significant when the P-value was < 0.05. Data are expressed as mean [95% confidence interval (CI)].
For the survival analysis, the nonparametric application Kaplan-Meier estimator was used, and the survivals of each group were compared with the log-rank test. Finally, an analysis was performed to assess the correlations between survival, sCaPP, and the variables studied. The Pearson correlation test was used to assess the correlations between survival in days and the sCaPP, blood pressure, urea, creatinine, potassium, total protein, and albumin.
Results
Nine dogs were diagnosed as chronically infected with Ehrlichia canis (3 staged in IRIS 1, 5 in IRIS 2, and 1 in IRIS 3) and 33 dogs were positively diagnosed with leishmaniasis (2 were in IRIS 1, 3 in IRIS 2, and 14 in IRIS 3). This high incidence of leishmaniasis is related to the geographical location of our hospital in southwest Spain, which is an endemic area for the disease.
Hematology and biochemistry
Comparison between sCaPP ≤ 70 mg2/dL2 and sCaPP > 70 mg2/dL2
Fifty-seven dogs included in the CKD group had a sCaPP ≤ 70 mg2/dL2; in the remaining 93 patients this value was > 70 mg2/dL2. These numbers remained unchanged when the sCaPP was used calculating total calcium instead of the corrected calcium (Table 1). However, sCaPP comparisons were calculated using total calcium. Dogs exhibiting sCaPP ≤ 70 mg2/dL2 and sCaPP > 70 mg2/dL2 showed statistically significant higher plasma concentrations of urea and creatinine compared to the control group (Table 1; P < 0.001). Dogs with sCaPP > 70 mg2/dL2 also showed an increase in the concentration of phosphorus and total protein concentration associated with lower albumin (P < 0.001; Table 1). When both groups were compared, a significant increase was observed in all parameters studied in the group of dogs with sCaPP > 70 mg2/dL2 compared to those with sCaPP ≤ 70 mg2/dL2; however, corrected and total calcium, sodium and potassium did not significantly vary between groups (Table 1).
Table 1.
Biochemical, urinary, and hematological values in control dogs or CKD affected dogs according to the sCaPP value.
| Control group (N = 25) | sCaPP ≤ 70 mg2/dL2 (N = 57) | sCaPP > 70 mg2/dL2 (N = 93) | |
|---|---|---|---|
| Urea (mmol/L) | 5.6 (4.80–6.41) | 23.36 (21.17–30.43)*** | 48.14 (44.49–54.79)***aaa |
| Creatinine (μmol/L) | 85.75 (81.33–91.05) | 266.09 (220.12–326.20)*** | 566.66 (518.03–678.93)***aaa |
| Phosphorus (mmol/L) | 1.23 (1.14–1.32) | 1.78 (1.76–2.48) | 4.72 (4.45–5.3)***aaa |
| Corrected calcium (mmol/L) | 2.46 (2.34–2.59) | 2.45 (2.30–2.56) | 2.36 (2.25–2.42) |
| Total calcium (mmol/L) | 2.47 (2.32–2.62) | 2.25 (2.09–2.42) | 2.32 (2.20–2.44) |
| sCaPP (mg2/dL2) | 38.94 (34.71–40.07) | 50.35 (41.35–62.1) | 136 (133.83–145.20)***aaa |
| Total protein (g/L) | 62.6 (60.0–65.20) | 64.3 (61.4–68.8) | 72.2 (69–76.4)***aaa |
| Albumin (g/L) | 35.4 (33.9–36.9) | 33.5 (28.5–32.9) | 30 (29.7–33.4)***aa |
| Sodium (mmol/L) | 149.97 (147.14–152.79) | 142.77 (135.69–151.73) | 144.94 (142.32–146.96) |
| Potassium (mmol/L) | 4.37 (4.10–4.63) | 4.43 (4.14–4.92) | 4.62 (4.3–4.86) |
| Urinary density | 1048.50 (1029.44–1067.56) | 1.018.37 (1016.59–1020.68)*** | 1.016.88 (1014.99–1017.94)*** |
| UP/C | 0.12 (0.08–0.15) | 6.57 (4.47–9.56) | 7.82 (3.15–8.93)** |
Values are expressed as mean (95% CI). A 1-way ANOVA followed by a Bonferroni post-hoc test was used to compare total protein and albumin (normal distribution), while a Kruskal-Wallis H-test was used to compare the rest, due to their non-Gaussian distribution. Significance was set at P < 0.05.
Significance value compared to control: (**) = P < 0.01; (***) = P < 0.001.
Significance value compared to sCaPP ≤ 70 mg2/dL2 group: (aa) = P < 0.01; (aaa) = P < 0.001.
Comparison between IRIS stages
Twenty-six dogs were staged as IRIS 1, with plasma creatinine concentrations less than 125 μmol/L; 27 dogs were staged as IRIS 2, with a plasma creatinine concentration between 125 and 180 μmol/L; 44 dogs were staged as IRIS 3, with a plasma creatinine concentration between 181 and 440 μmol/L; 53 dogs were staged as IRIS 4, with a plasma creatinine level > 440 μmol/L.
Analysis of the data obtained from dogs according to the IRIS classification showed a progressive increase in phosphorus concentration and sCaPP (Table 2). In this parameter, a statistically significant difference was observed between IRIS stages 3 and 4 and the control group (P < 0.001; F: 24.62; Table 2) and IRIS groups 1, 2, and 3 with IRIS 4 (P < 0.001; Table 2). In the IRIS 1 group, 100% of the dogs (26 of 26) had a sCaPP ≤ 70 mg2/dL2, in IRIS 2, 81.5% (22 of 27), in IRIS 3, 25% (11 of 44), and in IRIS 4, 7.5% (4 of 53).
Table 2.
Biochemical and hematological values in control dogs or CKD-affected dogs at IRIS stages 1 to 4.
| Control group (N = 25) | IRIS 1 (N = 26) | IRIS 2 (N = 27) | IRIS 3 (N = 44) | IRIS 4 (N = 53) | |
|---|---|---|---|---|---|
| Urea (mmol/L) | 5.60 (4.8–6.41) | 14.85 (9.81–19.88) | 19.82 (15.38–24.26)d | 33.26 (27.22–39.29)*d | 56.06 (50.86–61.26)*abc |
| Creatinine (μmol/L) | 85.75 (81.33–91.05) | 113.15 (91.94–135.25)d | 155.59 (144.98–167.08)d | 312.06 (289.96–334.16)*d | 751.42 (664.78–838.05)*abc |
| Phosphorus (mmol/L) | 1.23 (1.14–1.32) | 1.15 (0.96–1.94)d | 2.13 (1.54–2.72)d | 3.44 (2.83–4.04)*d | 4.93 (4.44–5.42)*abc |
| Corrected calcium (mmol/L) | 2.46 (2.34–2.59) | 2.45 (2.24–2.80) | 2.57 (2.35–2.79) | 2.58 (2.41–2.75) | 2.33 (2.20–2.46) |
| Total calcium (mmol/L) | 2.47 (2.32–2.62) | 2.43 (2.12–2.74) | 2.42 (2.2–2.63) | 2.32 (2.14–2.49) | 2.22 (2.06–2.37) |
| sCaPP (mg2/dL2) | 37.39 (34.71–40.07) | 42.83 (29.58–62.10)d | 63.18 (46.34–90.09)d | 95.57 (88.34–127.19)*d | 130.38 (125.16–153.52)*abc |
| Total protein (g/L) | 62.6 (60.00–65.20) | 63.70 (52.20–75.20) | 63.20 (56.40–70.00) | 71.60 (65.8–77.4) | 71.00 (67.70–74.30) |
| Albumin (g/L) | 35.4 (33.9–36.9) | 31.20 (25.7–36.6) | 30.80 (26.7–34.9) | 28.30 (25.9–30.7)*d | 33.70 (31.7–35.7)c |
| Sodium (mmol/L) | 149.97 (147.14–152.79) | 145.15 (112.75–177.55) | 139.03 (119.69–158.36) | 145.22 (142.19–148.24) | 145.27 (142.24–148.31) |
| Potassium (mmol/L) | 4.37 (4.1–4.63) | 4.14 (3.90–4.38) | 4.14 (3.63–4.65) | 4.52 (4.17–4.87) | 4.76 (4.39–5.13) |
| Urinary density | 1.048.50 (1029.44–1067.56) | 1.035 (1020.25–1031.75)bcd | 1.017 (1013.61–1020.39)*a | 1.017.92 (1016.07–1019.77)*a | 1.015.91(1014.13–1017.68)*a |
| UP/C | 0.12 (0.08–015) | 1.25 (0.87–0.25) | 6.03 (2.60–9.46) | 7.05 (3.02–11.09)* | 5.86 (2.93–8.78)* |
Values are expressed as mean (95% CI). A Kruskal-Wallis H-test was used to compare all the non-normally distributed values. For the normally distributed values (total protein and albumin), a 1-way ANOVA was used followed a Bonferroni post-hoc test. Significance was set at P < 0.05.
Significance compared to IRIS 1 group: a = P < 0.05.
Significance compared to IRIS 2 group: b = P < 0.05.
Significance compared to IRIS 3 group: c = P < 0.05.
Significance compared to IRIS 4 group: d = P < 0.05.
Significance value compared to control: (*) = P < 0.05.
Urinalysis
Comparison between sCaPP ≤ 70 mg2/dL2 and sCaPP > 70 mg2/dL2
Urine analysis showed statistically significant lower density in the sCaPP ≤ 70 mg2/dL2 and sCaPP > 70 mg2/dL2 groups compared to the control, but there were no differences for this parameter between the 2 groups. Dogs in both groups had proteinuria but it was significantly higher in dogs in the sCaPP > 70 mg2/dL2 group. The UP/C was significantly higher in dogs allocated to the sCaPP > 70 mg2/dL2 compared to the control group (Table 1; P < 0.01).
Comparison between IRIS stages
All the dogs in the CKD group (IRIS 2–4) had a USG < 1.030. Compared to the control group, UP/C was significantly higher for dogs with IRIS 3 or 4 disease but not for the dogs allocated to the IRIS 1 and 2 groups (Table 2; P > 0.05).
Blood pressure
Comparison between sCaPP ≤ 70 mg2/dL2 and sCaPP > 70 mg2/dL2
An increase in systolic blood pressure was observed in both groups (150.14 ± 33.07 mmHg and 162.46 ± 31.23 mmHg for sCaPP ≤ 70 mg2/dL2 and sCaPP > 70 mg2/dL2, respectively), but this was not statistically significant (P > 0.05; F: 0.08).
Comparison between IRIS stages
Dogs allocated to the IRIS 2 group had lower systolic blood pressure compared to the IRIS 3 and 4 groups but there were no statistically significant differences (P > 0.05; Table 2).
Mean survival
In our study, 96 dogs died due to the natural evolution of the CKD, 51 were euthanized at the owner’s request, and 3 dogs in the IRIS 1 group survived over 2 y and died due to causes not related to CKD.
Comparison between sCaPP ≤ 70 mg2/dL2 and sCaPP > 70 mg2/dL2
Regarding survival, a statistically significant difference (P < 0.001; χ2: 5.99) was observed between both groups (Figure 1). Lower mean survival was observed as the IRIS classification increased, showing a statistically significant difference between the IRIS 4 and IRIS 2 groups (Figure 2; P < 0.001; χ2: 19.51).
Figure 1.
Kaplan-Meier survival curve for dogs affected with CKD according to the sCaPP. Mean survival time and (95% CI) from sCaPP ≤ 70 mg2/dL2 group = 505.40 days (range: 113.31 to 539.52 days). Mean survival time from sCaPP > 70 mg2/dL2 group = 45.48 days (range: 5.8 to 149.0 days). Significance between groups was P < 0.001.
Figure 2.
Kaplan-Meier survival curve for dogs with CKD according to the IRIS 4 staging. Mean survival time (95% CI) from IRIS 2 group = 377.22 5 days (range: 23.10 to 731.34 days); IRIS 3 = 164.60 days (range: 29.88 to 299.32 days) and IRIS 4 = 48.83 days (range: 1 to 6 133.47 days). Significance between IRIS 2 and IRIS 4 P < 0.001. The survival study 7 was not performed in the IRIS 1 group because 24 dogs surpassed the observation 8 period (2 years).
Correlations
A positive correlation was observed in dogs with CKD between sCaPP with blood pressure (r = 0.343, P < 0.05), plasma urea concentration (r = 0.663, P < 0.05), creatinine (r = 0.588, P < 0.001), potassium (r = 0.243, P < 0.05), and total protein (r = 0.328, P < 0.001). There was a negative correlation with plasma albumin (r = 0.225, P < 0.05) and mean survival (r = 0.545, P < 0.001).
Discussion
It is well-established that when phosphorus intake remains constant in dogs with CKD, the lower GFR leads to its retention, resulting in hyperphosphatemia and RSHP (4). This hyperphosphatemia, together with normal or elevated serum calcium, will cause an increase in sCaPP (1). In the animals studied, there was an increase in the plasma concentration of phosphorus. This increase was markedly enhanced with the progression of CKD also resulting in a lower lifespan of the animals in the study, consistent with previous reports (20).
There have been more in-depth studies on the importance of phosphorus blood concentration in CKD than studies of sCaPP in humans (10), cats (11), and dogs (15,26). Increased sCaPP in these studies was suggested to be associated with faster progression of the disease and reduced lifespan of the patients (26,27). In agreement with these results and other studies, our data show a consistent relationship between enhanced sCaPP and severity of CKD (9,20,21,28). Compared to the control group, dogs with clinical and laboratory signs of CKD with sCaPP below the threshold value (≤ 70 mg2/dL2) had less evident alterations in markers of CKD than those with a sCaPP value > 70 mg2/dL2, indicating less severe CKD for the dogs in the ≤ 70 mg2/dL2 group (Table 2). These animals also had plasma phosphorus concentrations at the upper limit of the reference range or slightly above this threshold, indicating that the renal compensatory mechanisms were still able to maintain homeostasis of the ion. This corresponded with a marked lower survival in dogs with high sCaPP (> 70 mg2/dL2), compared to those with a sCaPP within the normal range (≤ 70 mg2/dL2), consistent with a recent report in which a smaller cohort of dogs was used (20). The results in our study demonstrate a progressively higher sCaPP coinciding with the progression of CKD. Although the mean values in dogs graded IRIS 1 and IRIS 2 are below the threshold established by Lippi et al (21) in 2014 (≤ 70 mg2/dL2), this value is slightly higher in dogs with IRIS 1 disease and becomes more evident in dogs with IRIS 2 CKD. The higher sCaPP values observed for dogs allocated to the IRIS 3 and 4 groups was associated with poorer prognosis as previously described (2,20), and these observations are similar to our results.
In human medicine, this relationship has been extensively studied (29) while in dogs, although some data have been provided (1,9,25), the results are contradictory. In the present study it was observed that starting from the IRIS 3 group, the value of sCaPP in most dogs was higher than 70 mg2/dL2, which is considered as a threshold above which metastatic calcification and a higher risk of mortality are expected. This highlights the importance of monitoring sCaPP and keeping it below the threshold of 70 mg2/dL2. Gear et al (30) in 2005 suggested that, although metastatic calcification also occurs in dogs, sCaPP is not useful as a prognostic indicator in CKD. However, Lippi et al (21) in 2014 identified a precise cutoff value (70 mg2/dL2) that predicted poor CKD prognosis and mortality, not considering creatinine concentration. The results from the dogs in the present study show the importance of the sCaPP value as an indicator of poor prognosis for CKD affected patients. Interestingly, in our study the existence of a direct correlation between sCaPP and plasma creatinine (r = 0.588; P < 0.001) was demonstrated while in the study by Lippi et al (21) this correlation could not be established. This difference may be explained by the greater number of patients in our study.
There are 2 main parameters associated with the severity of the CKD: increased systemic arterial pressure (4,31–33) and proteinuria (34). High blood pressure in the initial diagnosis of the CKD has been identified as a risk factor for the development of uremic crisis and mortality, and is associated with more severe proteinuria in dogs, cats, and humans with CKD (32). However, although increases were observed for both parameters (proteinuria and systolic blood pressure), only a low positive correlation between sCaPP and systolic blood pressure was observed.
Limitations of the present study include the use of total calcium measurements instead of the iCa which is the most accurate calcium measurement for the estimation of sCaPP (19). Nevertheless, our results demonstrate that total sCaPP using total calcium can predict dog lifespan in patients affected with CKD. Another limitation is that the impact of the different treatments for each patient was not taken in account. It should be noted that a specific renal diet and the prescription of phosphate binders was established for each patient allocated to the IRIS 2–4 groups the day on which CKD was diagnosed. Furthermore, a significant number of patients included in the present study were diagnosed with leishmaniasis, a specific treatment was prescribed and modified according to the progression of the disease.
In conclusion, according to the results obtained herein, sCaPP using 70 mg2/dL2 as threshold value predicts survival in canine CKD. All the dogs affected with CKD are at risk of progression, so it is important to provide the most accurate prognosis possible of the patients’ lifespan. This prognosis will directly influence the choice of treatment essential for delaying or preventing the progression of the disease as much as possible.
Acknowledgment
Beatriz Macias Garcia is funded by a Ramón y Cajal grant from the Spanish Ministry of Science, Innovation and Universities (RYC-2017-21545; AEI/FEDER/UE). CVJ
Footnotes
Use of this article is limited to a single copy for personal study. Anyone interested in obtaining reprints should contact the CVMA office (hbroughton@cvma-acmv.org) for additional copies or permission to use this material elsewhere.
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