5.3 Urinalysis & Specialized Biomarkers of Organ Toxicity
Key Takeaways
- Urine volume, specific gravity or osmolality, protein, glucose, occult blood, and casts remain the first-line renal function and injury screen; male-rat chronic progressive nephropathy is a proteinuria confounder, not automatic test-article glomerular disease.
- Urinary KIM-1, NGAL, and clusterin can rise after proximal tubular injury before BUN or creatinine move, because those chemistry tests are late GFR markers rather than site-specific tubular-injury tests.
- Cardiac troponin I or T reports cardiomyocyte injury more specifically than CK or AST; collection timing and capture stress still confound the result.
- Handbook III.A.1 H chemical-specific biomarkers of effect (for example red-cell acetylcholinesterase inhibition, methemoglobin, lead-associated ALA dehydratase change) document a known mechanism, not a generic chemistry rotor.
- Hours after last dose, fasting status, and spot versus timed urine collection change biomarker peaks; a 24-hour terminal sample can miss an injury marker that peaked earlier the same day.
Traditional urinalysis still decides many I.C.2 items
Before novel biomarkers, a competent toxicologist can already say a great deal from urine volume, specific gravity (SG) or osmolality, protein, glucose, occult blood, pH, and sediment (erythrocytes, leukocytes, crystals, casts). Independent OpenExamPrep material in this section treats that panel as the functional counterpart to kidney weights and histopathology, then adds specialized markers the 2026 handbook also expects you to recognize under hazard identification (III.A.1 H, chemical-specific biomarkers of effect) and under study interpretation (I.C.2).
Polyuria with low SG suggests a concentrating defect, solute diuresis, medullary washout, or a diabetes insipidus-like effect. Oliguria with high SG suggests prerenal azotemia or dehydration. Isosthenuria near 1.008–1.012 when the animal should be able to concentrate is a function finding: the tubules are not adjusting water. Metabolism-cage spill, leaking water valves (especially NHP), and diet wet-mash can fake polyuria; method notes belong in the interpretation, not the appendix no one reads.
Protein on a dipstick is a screen, not a quantified excretion rate. Glomerular injury leaks albumin; tubular injury leaks low-molecular-weight proteins the proximal tubule failed to reabsorb. Adult male rats develop chronic progressive nephropathy (CPN) with proteinuria that is background disease, not automatic test-article glomerulonephritis. Compare incidence and severity with concurrent controls of the same sex and age.
Glucose in urine means filtered load exceeded the reabsorptive threshold (hyperglycemia) or the proximal tubule cannot reabsorb a normal load (normoglycemic glucosuria, Fanconi-like toxicants). Always read urine glucose against the simultaneous serum glucose.
Occult blood on a dipstick does not distinguish erythrocytes, hemoglobin, and myoglobin. Centrifuge: a red pellet is hematuria; a red supernatant is pigment (hemoglobinuria or myoglobinuria). Pair pigmenturia with the CBC (hemolysis) or CK (rhabdomyolysis).
Casts form in the distal nephron. Hyaline casts can be bland (Tamm–Horsfall protein). Granular and cellular casts support tubular injury. Crystals may be diet, pH, or a poorly soluble metabolite—do not skip the vehicle and the pH when you see them.
Specialized kidney-injury biomarkers versus BUN
Kidney injury molecule-1 (KIM-1), neutrophil gelatinase-associated lipocalin (NGAL), and clusterin measured in urine can increase after proximal tubular injury before BUN or creatinine budge. BUN and creatinine are functional GFR markers. They often move only after a substantial fraction of filtering capacity is lost, and they are not site-specific. Dehydration, gastrointestinal bleed, and high-protein meals raise BUN without a tubule lesion.
Qualification work through consortia such as the Predictive Safety Testing Consortium supported several urinary proteins as voluntary additions in rat studies to detect tubular injury earlier than traditional chemistry. They do not replace histopathology, they do not make BUN obsolete, and they are not automatically validated in every species or assay platform you might use. Teach the three-way contrast:
- Traditional chemistry (BUN, creatinine, SDMA, cystatin C): function (GFR).
- Urinary KIM-1, NGAL, clusterin (and related qualified proteins): tubular cellular injury, often earlier.
- Histopathology: morphology and distribution.
Those three clocks can disagree. A day-3 KIM-1 increase with still-normal BUN and later proximal tubular necrosis is a coherent early-injury story. A BUN increase with concentrated urine, quiet KIM-1, and no tubular necrosis is often prerenal. Do not pick a winner in advance of the package.
Cardiac troponins
Cardiac troponin I (cTnI) and cardiac troponin T (cTnT) leak from injured cardiomyocytes. They are more cardiac-specific than CK or AST, which cannot localize heart versus skeletal muscle. Use troponins with heart weights, gross lesions, histopathology, and the safety-pharmacology context (ICH S7A/S7B live in a later chapter). Capture stress and difficult venipuncture can raise troponin modestly in NHP. A single terminal value without a time course is weaker than serial samples after a suspected cardiotoxic dose.
Urinary and serum metabolomics as emerging tools
Untargeted metabolomics (nuclear magnetic resonance or mass spectrometry of urine or serum) can show pattern shifts in energy intermediates, bile acids, or microbial metabolites. On the DABT examination, know them as emerging biomarkers of effect at a systems level. They are investigative. They are sensitive to diet, gut flora, fasting, and collection time. They are not a drop-in replacement for an OECD 408 clinical-pathology list. Overfitting a multivariate signature to eight rats is a real failure mode; independent confirmation and histopathology still rule.
Handbook III.A.1 H: chemical-specific biomarkers of effect
Hazard identification includes chemical-specific biomarkers of effect—evidence that a known molecular or physiologic target was perturbed, not merely that the parent chemical was present in blood (that would be a biomarker of exposure). Examples a general toxicologist should be able to name:
- Acetylcholinesterase (AChE) inhibition in red cells or brain after organophosphate or carbamate exposure (distinguish red-cell AChE from plasma butyrylcholinesterase; they do not always move together).
- Methemoglobin after oxidizing aryl amines or nitrites.
- Carboxyhemoglobin after carbon monoxide.
- Aminolevulinic acid dehydratase (ALA-D) inhibition or zinc protoporphyrin change after lead.
- Protein or DNA adducts for some electrophiles.
- Low-molecular-weight proteinuria such as β2-microglobulin in historical cadmium tubular injury work.
You choose these because of the chemistry, not because they print on the same rotor as ALT. Measuring AChE on a purely hepatotoxic solvent study is theater. Measuring only ALT on an organophosphate study misses the handbook’s chemical-specific-effect idea.
| Marker | What it usually indicates | Limitation |
|---|---|---|
| Urine volume + SG/osmolality | Concentrating ability, water balance | Cage spill, diet, leaking valves |
| Protein, glucose, blood, casts | Glomerular or tubular injury, hemorrhage, pigment | Rat CPN, contamination, dipstick non-specificity |
| KIM-1, NGAL, clusterin | Proximal tubular injury, often earlier than BUN | Assay, species, timing; not a histo replacement |
| Cardiac troponin I/T | Cardiomyocyte injury | Capture stress, hours after insult |
| Chemical-specific effect markers | Target perturbation of a known mechanism | Must match the chemistry (AChE is not a liver test) |
| Untargeted metabolomics | Emerging systems-level effect patterns | Diet, flora, overfitting, not a guideline default |
Timing of collection versus last dose
Biomarkers have kinetics. Troponin and KIM-1 may peak hours to a couple of days after injury and then decline even if daily dosing continues at a lower effective injury rate. A terminal bleed 24 hours after the last dose may miss an early peak you would have caught at 4–8 hours. Urinary markers depend on whether you collect a timed metabolism-cage urine or a spot sample at necropsy. Overnight fasting before the terminal bleed changes glucose, ketones, and some metabolites and must match the control group.
Protocol language should fix fasting status, hours after last dose, and collection method. Comparing a 4-hour postdose investigative sample from treated animals to a 24-hour terminal control sample is not a controlled experiment. For chemical-specific markers such as AChE, the time of nadir after a reversible carbamate is not the same as after a more persistent organophosphate; sampling “sometime on necropsy morning” can miss the pharmacodynamic trough.
Scenario
A 7-day rat study of an aminoglycoside-like tubular toxicant shows urinary KIM-1 and clusterin increased on day 3, BUN and creatinine still within concurrent-control ranges, concentrated urine, and, at necropsy on day 7, proximal tubular necrosis. That package illustrates early tubular-injury biomarkers beating GFR tests—not a reason to skip histopathology or to call BUN useless in every other design.
Contrast a high-dose group with BUN up, urine highly concentrated, quiet KIM-1, and no tubular necrosis: think prerenal (food refusal, water deprivation, or hemorrhage), not “the novel biomarkers failed.”
A third design measures cTnI only at necropsy 24 hours after the last oral dose and discards the 4-hour postdose bleeds. If myocyte injury peaked and cleared, the study can look “negative” for cardiotoxicity while histology of a missed time point would have told a different story. Timing is part of the biomarker, not an afterthought.
Traps
- Treating all male-rat proteinuria as test-article glomerular disease.
- Using BUN as a sensitive, site-specific tubular-injury test.
- Calling KIM-1 a GFR marker or a cardiac enzyme.
- Treating exposure biomonitoring (parent in plasma) as a biomarker of effect.
- Ignoring hours-after-last-dose when comparing groups.
Urinary KIM-1 and clusterin are increased on day 4 of a rat study, while BUN and creatinine remain within concurrent-control ranges. Histology later shows proximal tubular necrosis. What does this package illustrate?
A protocol collects cardiac troponin only at necropsy, 24 hours after the last oral dose, and discards 4-hour postdose investigative samples. Why might that design miss a real cardiotoxic effect?
Which example is a chemical-specific biomarker of effect in the handbook hazard-identification sense, rather than a generic clinical-pathology panel result?