Laboratory Tests and biomarkers Flashcards
Ideal renal biomarker characteristics
freely filtered by the glomerulus; do not undergo tubular reabsorption/secretion; Not secreted or metabolised elsewhere in the body.
Endogenous substances used for GFR estimation should also be produced at a constant rate and not affected by other biological factors such as age, diseases or diet
Direct measures of GFR
Plasma Inulin Clearance
Reference standard used in humans.
Plasma iohexol Clearance
Standard dose of iohexol given and then blood collected at 2,3,4 hours. GFR estimated from serum clearance normalised for body weight
Renal Scintigraphy
various injected radiotracers are extracted from the blood by the kidneys and emit radiation, particularly gamma radiation, that is detected by gamma cameras.
comparable to standard methods of GFR determination
SDMA Test principles advantages and limitations
Product of arginine protein degradation with no metabolic activity, detected by immunoassay where it competes with a labelled analyte for the same Ab → measurement is through how much of the Ab is able to bind the labelled conjugate
Subsequent studies comparing SDMA to sCr and GFR measurement (by iohexol clearance found a cut off of >18 improved specificity from 50 to 83% without compromising sensitivity (90%)
Increases with a >40% reduction in GFR (>30% in cats). Slight improvement compared to sCr for detection of early GFR reduction.
Likely to provide a more accurate measure of GFR in patients with sarcopenia compared to sCr
IRIS - To date there are no studies that have specifically evaluated whether SDMA is a superior marker for monitoring change in renal function or progression of disease in CKD patients although it could be hypothesised that this may be the case, particularly with continued weight and muscle loss with advancing disease and the known effect that this will have on Scr.
High index of individuality = potential for fluctuation within the patient. As such need to see a critical different of >6ug/dL change between samples to classify as significant
Extra-renal influences include:
- Young age may affect results (prior to onset of maturity)
- Upregulation in some human critical illness, small (underpowered) study in dogs did not find a difference
- Conditions that alter protein catabolism MAY impact production of SDMA (see hyperthyroidism)
- Neoplasia: conflicting reports, large burdens may result in non-renal production of SDMA. Elevations reported in lymphoma without azotaemia, and resolved with treatment of tumour burden.
- Diabetes: reported to lower SDMA compared to controls
- Hyperthyroidism - poor sensitivity for detection of risk for azotaemia development after Tx of disease - often low SDMA. Good specificity for development of azotaemia if elevated prior to treatment. Also report a lack of change in SDMA after treatment despite increases in sCr. Likely that hyperTH influences concentrations of SDMA relative to GFR.
Creatinine test principle and adv/disdv
breakdown of phosphocreatine in muscle, freely filtered by glomerulus and not reabsorbed making it a good marker of GFR.
JVIM 2019 comparison to iohexol clearance GFR reported 83% sens, 83% spec for detection of GFR decrease of >30%.
Readily available measurement, recommended in IRIS guidelines for CKD staging
→ for stage I and II sCr levels outlined by IRIS may still be within some lab RI
In IRIS AKI guidelines it is the trend in sCr, whether there is an increase in the first 48h, that is used to determine stage in early disease (ie those presenting after nephrotoxin ingestion and being monitored
Disadv:
Less specific than SDMA when using newer cutoff.
Insensitive to early decline in GFR
does not detect kidney damage when GFR has not been impacted
Reduced by low muscle mass - common in hyperthyroidism, cachexia from CHF or CKD
BUN adv and limitations
May correlate better with accumulation of uraemic toxins and clinical signs of uraemia than creatinine or SDMA
Not a useful GFR estimation due to tubular flow rates affecting reabsorption and production/excretion not constant
Can be reduced by non-renal factors: liver disease PSS, urea cycle enzyme deficiencies, failure), low protein diet, SIADH
Can Be increased by non-renal factors: increased absorption (high protein diet, GI bleeding, fever
sCys C principle and adv/disadv
Non-glycosylated protein produced by all nucleated cells at constant rate. Freely filtered through glomerulus.
Reabsorbed in proximal convoluted tubule → fully metabolised
No secretion by tubular cells
May be more sensitive for detection of stage I CKD
Can be used as a GFR marker even though is reabsorbed because it is not secreted after reabsorption and thus concentrations mainly affected by GFR in dogs.
JVIM 2019 - dogs specificity 72% compared to 92% creatinine and 87% SDMA by comparison to GFR measured by scintigrapjhy
No cat specific assay available - higher in CKD cats but some overlap with healthy cats. Not different for different IRIS stages in cats.
Hyperthyroidism increases sCysC in cats independent of renal function
When to measure FGF 23 - IRIS guidelines
Once plasma phosphate concentration falls into the target range for the stage of CKD, measurement of plasma FGF23 concentration would then be valuable to continue to monitor the response to treatment. If FGF23 plateaus well outside the recommended concentration, then additional measures to restrict intestinal phosphate absorption should be instituted
In CKD patients where plasma phosphate concentration is outside the IRIS target range for their IRIS stage there is no need to measure plasma FGF23 concentration as plasma phosphate can be used to monitor response to treatment
Biomarkers of tubular dysfunction (4)
Urine albumin:Cr ratio - limited availability, likely not superior to UPCR
Neutrophil Gelatinase Associated Lipocalin (NGAL) - presence in urine is indicative of tubular cell damage (as serum levels are reabsorbed if filtered). Affected by the presence of LUTI infections and seems to have wide inter-individual variation in healthy dogs. Conflicting results on use as early damage biomarker
Liver-type Fatty Acid Binding Protein
Produced in hepatocytes and proximal tubular cells
Circulating LFABP is filtered but reabsorbed.
Locally produced L-FABP plays a major role in fatty acid homeostasis, resulting in fatty acid catabolism that provides energy to proximal tubular cells. Ischemic injury and oxidative stress of the proximal tubules result in L-FABP excretion into the urine. biomarker of early tubular damage that can reflect the severity of tubulointerstitial injury, detect AKI. Limited evidence - seems to be overlap in some healthy cats and not akll CKD have increased levels. MAy be better in AKI
uCysB- new Idexx test. Intracellular protein with limited concentration in the circulation, rupture of tubular cells to have increased levels in urine as is completely resorbed. So is an indicator of tubular damage. May be useful as an active kidney injury marker- may be assocaited with risk of progression of CKD
remains to be determined if current therapeutic interventions can slow this progression. But could be used to study interventions in the future.
Despite the high prevalence of CKD in dogs and cats, there are no definitions or established guidelines to differentiate animals with slow vs moderate or high progression rate. uCysB may originate from cells other than renal tubular as it is ubiquitous in the body especially as not fully evaluated in other diseases.
Idexx recommendations for use of uCysB
1) You suspect a toxin exposure and are looking to confirm active injury when
functional markers may not have changed yet.
2) You are monitoring treatment and recovery from an active injury event.
3) You need to know if a dog with IRIS CKD Stage 1 is progressing.
4) You have a patient that may be at higher risk when taking NSAIDs.
5) You’re monitoring the effect on kidneys during a shock event, such as heat stroke or acute pancreatitis.
6) You’re assessing the impact of a hypotensive event during anaesthesia to determine whether kidney injury is ongoing
Publications on the performance of Sedivue
1) 2 different analysers 70% accuracy
false positive for bacteriuria common, especially if pyuria present
recommend manual evaluation for bacteria or review of images.
2) idexx funded study reported sensitivie >80% for all parameters except epithelial cells. and specificity high
Recommended renal biopsy technique
JVIM 2018 - Number of glomeruli per core specimen was significantly associated with needle gauge, dog age, serum creatinine concentration, and degree of proteinuria
ultrasound-guided biopsy with a 16-gauge needle should maximize the diagnostic quality of renal biopsy specimens from dogs with suspected kidney disease
Causes of normoglycemic glucosuria
primary renal glucosuria, fanconi syndrome (due to aminoglycosides; hypoxia; C. perfringens infection) or inherited tubular defects.
False + in Ciprofloxacin, penicillin and cephalosporins; cystitis; hydrogen peroxide; formaldehyde; expired test strips
Significance of bilirubinuria
Bilirubinuria (in excess for the USG) is expected to precede bilirubinemia and may maintain total (and direct) bilirubin concentrations within reference intervals in early cholestasis
Bilirubinuria is seen with Intravascular RBC haemolysis, intrahepatic disease, and cholestatic disease/biliary obstruction
Types of urinary casts (6)
Hyaline - low numbers normal, high numbers in prerenal or renal proteinuria
Cellular - ischemia, infarction, or nephrotoxicity cause degeneration and necrosis of tubular epithelial cells
Granular - form as a stage in the degeneration of cellular casts
Fatty - tubular degeneration similar to granular casts
Waxy - suggests urine spent extended period within tubules and prior tubular injury. They are the final stage of cast degeneration
RBC or WBC casts - intrarenal haemorrhage
Proposed mechanism for hypercoagulability in glomerular disease
Mechanism underlying hypercoagulable state is poorly understood, and probably multifactorial, but theories include: loss of antithrombin (similar MW and charge to albumin); an increase in plasma procoagulant cofactors, factors V and VIII, and fibrinogen levels; increased platelet reactivity; possible role for vessel wall involvement (endothelial dysfunction)
JVECCS curative domain recommend use of antithrombotics/anticoagulants in PLN due to LoE that it is associated with increased risk. Venous TE occur so likely anti-coag Tx are going to be more effective.
