Renal Function, Urinalysis, Glucose, and Lipid Metabolism

Flashcards reviewing urinalysis, renal function tests, nonprotein nitrogen compounds, osmolality, glucose metabolism, diabetes pathophysiology, and lipid profile measurements.

Which compound constitutes the largest percentage of total nonprotein nitrogen (NPN) in plasma and excreted nitrogen in urine?

Urea accounts for $45-50\%$ of total plasma NPN and $86.0\%$ of excreted nitrogen in urine.

What are the primary clinical purposes of the urine protein-to-creatinine ratio (UPCR) test?

It checks for proteinuria (an early marker of glomerular damage) and serves three main purposes: screening patients with CKD risk factors, diagnosing suspected kidney involvement, and monitoring existing kidney conditions or drug toxicity.

What are the clinical cutoffs and interpretations for the urine protein-to-creatinine ratio (UPCR)?

Normal is $0.148\,\text{mg/mg}$ creat or less in men and $0.184\,\text{mg/mg}$ creat or less in women; mildly high is $0.2-3.0\,\text{mg/mg}$ (indicating possible kidney stress); and nephrotic range is $>3.0\,\text{mg/mg}$ (indicating significant protein loss).

What major endocrine hormones are produced or activated by the kidneys?

Renin, erythropoietin (EPO), and active Vitamin D3 (which helps absorb calcium).

In macroscopic urinalysis, what do white foam and yellow foam indicate?

White foam indicates increased protein (albumin), whereas yellow foam indicates bilirubin and potential renal failure.

What abnormal quantitative thresholds are used during 40x microscopic urinalysis for red blood cells and white blood cells?

RBC count $>2$ per high-power field and WBC count $>5$ per high-power field are considered abnormal.

Under what physiological conditions do urinary casts form, and what protein constitutes their matrix?

Casts form with urinary stasis, decreased $\text{pH}$, and increased protein concentration; they are composed of Tamm-Horsfall protein.

Which urinary crystals are typically observed in acidic urine ($\text{pH} < 7$)?

Calcium oxalate, amorphous urates, uric acid, cholesterol, and cystine.

Which urinary crystals are typically found in alkaline urine ($\text{pH} > 7$)?

Amorphous phosphates, calcium carbonate, triple phosphate, and ammonium biurate.

Which five crystals found in urine are classified as pathological crystals?

Leucine, tyrosine, cystine, sulfonamides, and cholesterol.

What analytes are represented by the acronym LIPAFAM, which are most severely affected by hemolysis interferents?

LDH, Inorganic phosphate, Potassium, AST (aspartate aminotransferase), Folic acid, ALT, and Magnesium.

Why must sodium fluoride and citrate tubes be avoided when collecting samples for urea determination?

Sodium fluoride and citrate interfere directly with the urease enzyme used in clinical assay methods.

How are urea-to-creatinine ratios classified to differentiate pre-renal, renal, and post-renal conditions?

A ratio $>80$ indicates a pre-renal condition; a ratio between $50-80$ indicates an intrinsic renal pattern; and a ratio $<50$ indicates a post-renal obstruction or decreased urea production.

What is the difference between azotemia and uremia?

Azotemia refers strictly to elevated levels of urea in the blood, whereas uremia refers to elevated urea in the blood accompanied by renal failure.

What is the reference range for plasma urea?

$2-7\,\text{mmol/L}$.

What metabolic process produces uric acid, and where does the majority of its elimination occur?

Uric acid is the end product of purine (adenine and guanine) nucleic acid metabolism; approximately $70\%$ is eliminated via renal excretion and the remaining $30\%$ enters the GI tract to be degraded by bacterial enzymes.

At what plasma concentration does monosodium urate precipitate to form urate crystals?

At concentrations greater than $6.8\,\text{mg/dL}$.

What is the reference range for plasma uric acid?

$210-430\,\mu\text{mol/L}$.

From which amino acids is creatine synthesized in the liver prior to forming creatinine?

Arginine, glycine, and methionine.

Why is creatinine clearance or eGFR preferred over plasma urea for assessing glomerular filtration?

Creatinine is released at a constant rate proportional to muscle mass, is completely filtered by the glomerulus, and is not reabsorbed by the proximal tubules.

What is the reference range for plasma creatinine?

$50-100\,\mu\text{mol/L}$.

What special specimen handling protocol is mandatory when testing for plasma ammonia?

Collect in heparin or EDTA tubes (never SST), place immediately on ice to slow metabolism, centrifuge at $4\,^{\circ}\text{C}$ within $20\,\text{minutes}$, separate plasma from cells immediately, and avoid any hemolysis.

What is the reference range for plasma ammonia?

$11-35\,\mu\text{mol/L}$.

What hallmark findings differentiate nephrotic syndrome from acute glomerulonephritis?

Nephrotic syndrome is characterized by massive proteinuria ($>3.5\,\text{g/day}$), hypoalbuminemia, severe edema, hyperlipidemia, and oval fat bodies; acute glomerulonephritis exhibits hematuria, RBC casts, and proteinuria ($<3\,\text{g/day}$).

What formula is used to calculate serum osmolality?

$\text{Calculated Osmolality} = (2 \times [\text{Na}]) + [\text{urea}] + [\text{glucose}]$.

What does an osmolality gap greater than $10\,\text{mOsmol/kg}$ indicate?

It indicates the presence of unmeasured exogenous volatile substances or organic solvents, such as alcohols or acetone.

Why is vapor pressure osmometry unsuitable for emergency department patients with suspected poisoning?

Vapor pressure osmometry does not measure volatile compounds such as alcohols.

How does Type 1 diabetes differ from Type 2 diabetes regarding ketosis and etiology?

Type 1 diabetes involves absolute insulin deficiency, affects mostly adolescents, and is ketosis-prone; Type 2 diabetes involves insulin resistance without insulin deficiency, affects mostly older adults ($>50$), and is not prone to ketosis.

What is the renal threshold concentration at which glucose spills into the urine?

$10\,\text{mmol/L}$.

Why does hyperkalemia develop in patients with diabetic ketoacidosis (DKA)?

Excess hydrogen ions ($\text{H}^+$) enter RBCs to be buffered by hemoglobin, creating a positive charge that forces intracellular potassium ($\text{K}^+$) out into the plasma.

Based on this figure demonstrating cation and anion concentrations, what is the formula for the anion gap and how does it change in lactic acidosis?

The formula is $([\text{Na}^+] + [\text{K}^+]) - ([\text{Cl}^-] + [\text{HCO}_3^-])$; in lactic acidosis, unmeasured organic acids increase (from $3$ to $9$) and bicarbonate drops (from $28$ to $22$), increasing the anion gap from $15$ to $21$.

What is the average rate of glycolysis in uncentrifuged whole blood at room temperature?

Approximately $5\%$ to $7\%$ per hour, or about $0.5$ to $1.0\,\text{mmol/L}$ per hour.

Why is a gray-top tube containing sodium fluoride and potassium oxalate chosen when processing delays are anticipated for glucose testing?

Sodium fluoride inhibits glycolysis by red blood cells, stabilizing glucose concentrations for up to $4\,\text{hours}$ and beyond.

What is the expected concentration of glucose in cerebrospinal fluid (CSF) relative to plasma?

CSF glucose is approximately $60\%$ of the plasma glucose concentration.

What time period of glycemic control does Hemoglobin A1c (HbA1c) represent, and what is its normal reference value?

HbA1c reflects average glucose stability over $8-12\,\text{weeks}$, with normal values being $4-6\%$ ($<6\%$ is good control).

What are the reference ranges for fasting glucose, random glucose, critical low glucose, and critical high glucose?

Fasting: $4-6\,\text{mmol/L}$; Random: $4-8\,\text{mmol/L}$; Critical low: $<2.0\,\text{mmol/L}$; Critical high: $>28\,\text{mmol/L}$.

What is the Friedewald equation for calculating LDL cholesterol in SI units, and when is it invalid?

$\text{LDL} = \text{Total Cholesterol} - \text{HDL} - \left(\frac{\text{Triglycerides}}{2.2}\right)$; it is invalid if triglycerides exceed $4.51\,\text{mmol/L}$.

What causes pseudohyponatremia in severely lipemic specimens, and how can it be resolved?

The electrolyte exclusion effect displaces water and sodium during indirect ISE measurement; it is resolved by ultracentrifugation to clear lipids or by analyzing the sample on a direct ISE analyzer (such as a blood gas analyzer).