What it is:The skill of reading a BMP, CMP, CBC, or ABG and turning raw numbers into a clinical story: is this patient dehydrated, in kidney failure, bleeding, acidotic, or just fine? Every rotation starts with you scanning a lab panel before you've even seen the patient.
The core problem:A single lab value out of context is almost useless. The same potassium of 5.4 mEq/L means nothing in a hemolyzed sample and means an emergency in a patient on spironolactone with an AKI. You always need three things: the number, the trend, and the clinical picture.
What you do about it:Learn the normal ranges cold, know which values need correction before you trust them (calcium, weight-based renal function), and know which combinations of labs localize a problem (BUN:Cr ratio, FENa, the ABG triad).
Treat every lab value as a clue, not a diagnosis. The exam (and rounds) will hand you a panel and ask you to name the pattern before naming the disease. Build the habit now: scan electrolytes, then renal function, then acid-base, then hematology, in that order, every time.
These three orders cover the vast majority of what you'll interpret day to day. Knowing what's bundled into each saves you from ordering redundant tests and helps you spot what's missing from a chart.
| Panel | Also called | Contains |
|---|---|---|
| Basic Metabolic Panel (BMP) | Chem-7 | Sodium, potassium, chloride, bicarbonate, glucose, BUN, serum creatinine (+/- calcium) |
| Complete Metabolic Panel (CMP) | Chem-14 | Everything in the BMP, plus liver function tests, magnesium, phosphate, and calcium if not already included |
| CBC with differential | - | Hemoglobin, hematocrit, RBC count, WBC count with differential (neutrophils, bands, eosinophils, basophils, lymphocytes, monocytes), platelets |
BMP is not the same as a renal panel and CMP is not the same as an LFT panel.A CMP is just a BMP with liver tests, magnesium, and phosphate bolted on. If someone says "check a CMP" because they're worried about the kidneys, they've actually ordered more than they need, and if they say it because they're worried about the liver, they got what they needed plus extras.
Reference ranges below are pulled from two separate course sources and they don't match perfectly (chloride 98–107 vs 95–105, bicarbonate 23–30 vs 22–28, BUN 8–23 vs 8–20). That's normal. Every lab has its own reference range depending on its assay and population. Learn the ballpark, and always read off whatever range is printed next to the value on the actual chart.
Don't just memorize ranges. Know each electrolyte's job, because that's what tells you why a derangement causes the symptoms it causes.
| Electrolyte | Normal range | Job | Critical values |
|---|---|---|---|
| Sodium (Na⁺) | 135–145 mEq/L | Most abundant extracellular cation; main driver of body water balance | <125 (hypo) · >155 (hyper) |
| Potassium (K⁺) | 3.5–5 mEq/L | Primary intracellular cation; muscle and nerve excitability, cardiac conduction | <3 (hypo) · >6 (hyper) |
| Chloride (Cl⁻) | 98–107 mEq/L | Most abundant extracellular anion; passively balances cations and maintains osmolarity | Tracks fluid and acid-base status, not usually a standalone emergency |
| Bicarbonate (HCO₃⁻) | 23–30 mEq/L | Buffers blood pH; used interchangeably with CO₂ on a chem panel | Low = acidosis, high = alkalosis |
| Calcium (Ca²⁺, total) | 8.5–10.5 mg/dL | Neuromuscular signaling, clotting, bone | <6.5 (hypo) · >13.5 (hyper) |
| Phosphate (PO₄³⁻) | 2.5–4.5 mg/dL | Intracellular anion; ATP, phospholipid membranes, RNA | Moves inversely with calcium |
| Magnesium (Mg²⁺) | 1.7–2.2 mg/dL | Intracellular cation; cofactor tied tightly to potassium handling | - |
Refractory hypokalemia is a magnesium problem until proven otherwise.If you're replacing potassium and the level won't budge, check magnesium and replace it first. Low magnesium drives renal potassium wasting, so you're pouring potassium into a leaking bucket until the magnesium is fixed.
Calcium is 99.5% bone.Of what's left in serum, roughly half is free (ionized, the biologically active form) and the rest is bound to albumin. That means a low albumin makes total calcium look falsely low even though the ionized fraction, the part that actually matters physiologically, may be normal. That's why you never trust a total calcium in a hypoalbuminemic patient without correcting it first, see Correction Calculations.
Phosphate and calcium move in opposite directionsbecause of how the body handles calcium-phosphate product. Hyperphosphatemia is a classic AKI and CKD complication precisely because failing kidneys can't excrete phosphate, and the resulting high calcium-phosphate product drives vascular and soft-tissue calcification.
This is the single most practically important skill in this whole document. Nearly every renally-cleared drug dose in your future career depends on getting this right.
Creatinineis the breakdown product of creatine and creatine phosphate, byproducts of muscle metabolism. It's cleared almost entirely by the kidney (85–90% glomerular filtration, 10–15% tubular secretion), so it's used as a surrogate for GFR. In a patient with stable kidney function, SCr sits at a steady-state baseline. The catch: when kidney function suddenly drops, SCr lagsbehind the real decline in filtration by a day or more, because it takes time for creatinine to accumulate to a new steady state. A "normal" SCr checked too early after an insult can be falsely reassuring.
BUNis the nitrogenous end product of protein metabolism in the liver, filtered and partially reabsorbed by the kidney. On its own it's a mediocre kidney marker, since it rises with a high protein diet, GI bleeding, corticosteroids and tetracyclines, and falls with malnutrition or severe liver disease. Its real value is relative to creatinine, not alone.
BUN:SCr ≥ 20:1 points to volume depletion (prerenal).Urea is passively reabsorbed along with water in the proximal tubule, but creatinine isn't affected by water reabsorption. So when the kidney is avidly reabsorbing water and sodium (low flow states), urea comes along for the ride and BUN rises disproportionately to creatinine. A ratio under 20:1 with a rising creatinine points away from a pure prerenal picture.
You need this equation memorized, cold, no calculator reference allowed on exam day for this one specifically.
CrCl (mL/min) = [(140 − age) × weight (kg) × (0.85 if female)] / (72 × SCr)
Age in years, weight in kg, SCr in mg/dL. Use actual body weightif it's below ideal body weight. If actual body weight is more than 30% above ideal, use an adjusted body weightinstead (formulas in Correction Calculations).
Male, age 56, weight 73 kg, height 68 in, SCr 1.2 mg/dL (at his own baseline).
IBW = 50 kg + (2.3 kg × inches over 5 ft) = 50 + (2.3 × 8) = 68.4 kg. Actual body weight (73 kg) isn't more than 30% above that, so you'd use actual body weight here, not the adjusted formula.
CrCl = [(140 − 56) × 68.4] / (72 × 1.2) = ≈ 66.5 mL/min
If this were a female patient of the same age, weight, and SCr, you'd multiply the numerator by 0.85, dropping the estimate by 15%, all else equal. Same kidneys, different estimate, because the equation is a population-based approximation, not a direct measurement.
Cockcroft-Gault assumes steady-state creatinine. It is not accurate in acute kidney injury, defined here as a sudden SCr increase of 0.3 mg/dL or 1.5x baseline, because the SCr hasn't caught up to the true drop in filtration yet. Using a fresh, still-rising SCr in the equation will overestimate how much kidney function the patient actually has left. It's also less accurate in very young patients and wasn't derived in a diverse population (the original 1976 cohort was 249 White men).
Cockcroft-Gault estimates creatinine clearancein mL/min; the eGFR equations (MDRD, CKD-EPI) estimate glomerular filtration ratenormalized to body surface area, in mL/min/1.73m². They're used for diagnosing and staging kidney disease and long-term monitoring, not usually for drug dosing, where CrCl by Cockcroft-Gault remains the standard most package inserts were built on.
| Equation | Year | Inputs | Note |
|---|---|---|---|
| MDRD | 1999/2007 | SCr, age, sex, race | Derived in patients with eGFR <60; included a race adjustment |
| CKD-EPI (creatinine) | 2009 | SCr, age, sex, race | More accurate than MDRD at higher GFRs; also race-adjusted |
| CKD-EPI (creatinine) | 2021 | SCr, age, sex | Race-freereformulation, now the preferred equation |
| CKD-EPI (cystatin C) | 2012 | Cystatin C, age, sex | Useful when SCr is unreliable (e.g. low muscle mass) |
Race is a social construct, not a biological one, and including a fixed multiplier for "Black" vs "non-Black" patients in MDRD and the 2009 CKD-EPI equation systematically raisedthe calculated eGFR for Black patients, up to roughly 16–21%, which could delay dialysis planning, transplant referral, or drug dose adjustment. That's a large part of why the field moved to the race-free 2021 CKD-EPI equation. Getting a number out of an equation doesn't mean the number is correct for that patient.
| Serum creatinine | Cystatin C | |
|---|---|---|
| Source | Muscle metabolism | All nucleated cells |
| Cleared by | Glomerular filtration (85–90%) + tubular secretion (10–15%) | Glomerular filtration (100%) |
| Half-life | ≈4 hours | ≈1.5 hours |
| Falsely lowered by | Eating disorders, amputation, muscle wasting, frailty, advanced age | - |
| Falsely raised by | Extreme muscle bulk; drugs that block tubular secretion (trimethoprim, cimetidine, fenofibrate, cobicistat, dolutegravir, TKIs) | Obesity, smoking, systemic inflammation, high-dose steroids |
Cystatin C's shorter half-life makes it react faster to a real change in kidney function, which is exactly why it's the better tool in a patient whose creatinine is confounded (very low muscle mass, an amputee, someone on trimethoprim) or whose SCr hasn't caught up yet to an acute change. BUN can help support the picture but isn't accurate as a stand-alone GFR biomarker on its own.
Before you diagnose or stage AKI: assess and optimize volume status, and rule out obstruction. A "worsening creatinine" that's actually dehydration or a kinked foley is not AKI in the way that changes your workup.
| Stage | Creatinine criteria | Urine output |
|---|---|---|
| 1 | ↑ SCr ≥0.3 mg/dL, or 1.5–1.9x baseline | <0.5 mL/kg/hr × 6–12 hr |
| 2 | SCr 2–2.9x baseline | <0.5 mL/kg/hr × 12–24 hr |
| 3 | SCr ≥3x baseline, or ≥4 mg/dL, or KRT initiated | <0.3 mL/kg/hr × ≥24 hr, or anuria ≥12 hr |
AKI is defined by any one of: SCr rise ≥0.3 mg/dL within 48 hours, SCr rise ≥1.5x baseline within the prior 7 days, or urine output <0.5 mL/kg/hr for 6 hours. Any one criterion is enough, they don't all have to be met at once.
| Phase | What's happening | Typical duration |
|---|---|---|
| Onset | Triggering event (blood loss, burns, fluid loss); symptoms of the underlying cause may show | Hours to days |
| Oliguric | Progressive decline, falling urine output, rising urea and creatinine; risk of fluid overload, hyperkalemia, metabolic acidosis, uremia | Usually <2 weeks |
| Diuretic | Underlying cause resolved, GFR recovers, urine output climbs, but tubular reabsorption is still scarred, risking dehydration and electrolyte loss | ≈3 weeks |
| Recovery | Fluid, electrolytes, and kidney function normalize | Months to a year |
Don't assume rising urine output means the kidney crisis is over. The tubules are still damaged and can't reabsorb properly, so patients can swing straight from oliguric fluid overload into diuretic-phase dehydration, hyponatremia, and hypokalemia. Watch electrolytes just as closely on the way out as on the way in.
| Prerenal | ATN | AIN | Postrenal | |
|---|---|---|---|---|
| Typical cause | Dehydration, hypoperfusion | Ischemia, nephrotoxins | Drug reaction, infection, autoimmune | Obstruction |
| BUN:Cr | >20:1 | <20:1 | <20:1 | Variable |
| Urine Na | <20 mEq/L | >20 mEq/L | Variable | Variable |
| FENa | <1% | Variable | Variable | Variable |
| Urine osmolality | >500 mOsm/kg | 250–300 | Variable | <400 |
| Sediment | Benign or hyaline casts | Granular ("muddy brown") casts | WBCs, WBC casts, +/- eosinophils | Normal, or RBC/WBC/crystals |
These are patterns to guide your thinking, not hard thresholds you can apply blindly. Real patients overlap categories.
Fractional excretion of sodium= [(Urine Na × Serum Cr) / (Urine Cr × Serum Na)] × 100. It measures what percent of filtered sodium the kidney is excreting. <1% = prerenal(kidney is desperately conserving sodium because it's underperfused). >2% = intrinsic(tubules are damaged and can't reabsorb sodium properly, so it spills into the urine). 1–2% is indeterminate. It's most useful in an oliguric patient, and it's unreliable if the patient has already received diuretics, since diuretics force sodium excretion regardless of the underlying cause. In that situation use FEUreainstead (swap sodium for urea in the formula): ≤35% suggests prerenal, >50% suggests intrinsic.
Acute tubular necrosis (ATN)is direct injury to the tubular cells, usually from ischemia (hypotension, sepsis) or nephrotoxins (aminoglycosides, amphotericin B, IV contrast, rhabdomyolysis, tumor lysis). Two things happen histologically: cell sloughing and cast/debris occlusion of the tubule lumen.
Acute interstitial nephritis (AIN)is inflammation of the tissue between the tubules, usually a drug hypersensitivity reaction. Classic culprits: NSAIDs, PPIs, beta-lactams (penicillins, cephalosporins), thiazides, and loop diuretics like furosemide. Only about a third of patients actually have eosinophiluria, so its absence doesn't rule AIN out; oliguria and a rising SCr may be the only clues you get.
Postrenal AKIneeds bilateralobstruction to cause a rise in SCr (or unilateral obstruction in someone with only one functioning kidney). A single obstructed ureter with a healthy contralateral kidney usually won't move the creatinine, because the other kidney compensates.
Metabolic:acidosis, hyperkalemia, hypocalcemia, hyperphosphatemia, uremia. CV:fluid overload, hypertension, arrhythmia, pericarditis. Heme:anemia, coagulation abnormalities. Neuro:encephalopathy, seizures. GI:nausea, bleeding. Infection riskis elevated across the board.
| Test | Normal range | What it tells you |
|---|---|---|
| AST | <35 units/L | Present in liver, RBCs, skeletal muscle, brain, kidney, heart. Not liver-specific. |
| ALT | <35 units/L | Mostly liver-specific |
| ALP | 41–133 IU/L | Bone, liver, intestine, kidney, placenta. Nonspecific for liver alone. |
| Bilirubin, total | 0.3–1 mg/dL | Indirect (unconjugated) 0.2–0.7, direct (conjugated) 0.1–0.3 |
| Albumin | 3.4–4.7 g/dL | Osmotic pressure, drug and hormone binding |
An AST:ALT ratio of about 2:1points toward alcoholic hepatitis. AST and ALT climbing into the 1000s units/Lis a classic pattern for drug-induced hepatotoxicity, not the more modest elevations you see with chronic viral hepatitis or fatty liver.
ALP is a poor stand-alone liver marker because it's just as elevated by bone turnover (fractures, rickets, vitamin D abnormalities), pediatric bone growth, and third-trimester pregnancy. Pair it with GGT or the transaminase pattern before blaming the liver.
Chronically low albumin signals severe liver dysfunction, protein loss (nephrotic syndrome, protein-losing enteropathy), or poor nutritional status, but for a pharmacist it also matters directly: low albumin means less protein binding, which changes free drug concentrations for highly protein-bound drugs like phenytoin. A "low" total phenytoin level in a hypoalbuminemic patient can still represent a therapeutic free level. Same logic as the corrected calcium problem in Correction Calculations, low albumin distorts the number you're reading.
| Test | Normal range | Notes |
|---|---|---|
| Hemoglobin | Men 13.6–17.5 g/dL · Women 12–15.3 g/dL | Oxygen-carrying capacity; low in anemia |
| Hematocrit | Men 39–49% · Women 35–45% | Roughly 3x the hemoglobin value; sanity-check the two against each other |
| RBC count | Men 4.3–6 ×10&sup6;/µL · Women 3.5–5.5 ×10&sup6;/µL | Median RBC lifespan ≈120 days |
| WBC | 4.5–11 ×10³/µL | See differential below |
| Platelets | 150–450 ×10³/µL | Thrombocytopenia <50, thrombocythemia usually >800 |
| Cell | Normal % | Rises with |
|---|---|---|
| Neutrophils | 50–70% | Bacterial infection |
| Bands (immature neutrophils) | 0–5% | Acute infection ("left shift") |
| Lymphocytes | 20–40% | Viral infection, cellular immunity |
| Monocytes | 2–6% | Recovering from infection, chronic conditions |
| Eosinophils | 1–4% | Allergy, asthma, parasitic infection, drug reactions |
| Basophils | 0–1% | Hypersensitivity, chronic inflammation |
Thrombocytopenia <50 ×10³/µLis your line for real bleeding risk concern; think drug-induced causes, DIC, aplastic anemia, leukemia. Thrombocythemia >800 ×10³/µLis most often reactive (severe physical stress, infection) but can also come from splenectomy, trauma, cirrhosis, or chronic pancreatitis.
A large variation in RBC size (a sign that shows up as elevated RDW on a full CBC) is a classic tell for iron deficiency anemia, since new cells being made under iron restriction come out smaller and more variably sized than the older population.
The whole system runs on one buffer equation: CO₂ + H₂O ↔ H₂CO₃ ↔ HCO₃⁻ + H⁺. Everything you interpret on an ABG is really just reading which side of that equation is out of balance.
| Value | Normal | Critical |
|---|---|---|
| pH | 7.35–7.45 | <7.25 or >7.55 |
| pCO₂ | 4.5–6.0 kPa (36–44 mmHg) | - |
| HCO₃⁻ (arterial) | 22–26 mEq/L | - |
| Base excess | −2 to +2 mmol/L | - |
pH always tells you the direction first.Below 7.35 is acidosis, above 7.45 is alkalosis. Then figure out whether the driver is metabolic (HCO₃⁻ moved) or respiratory (pCO₂ moved). Base excess is just a surrogate for how much bicarbonate is present relative to normal, and it moves the same direction HCO₃⁻ does.
| Disorder | pH | pCO₂ | HCO₃⁻ |
|---|---|---|---|
| Respiratory acidosis | ↓ | ↑ | normal |
| Respiratory alkalosis | ↑ | ↓ | normal |
| Metabolic acidosis | ↓ | normal | ↓ |
| Metabolic alkalosis | ↑ | normal | ↑ |
The body always tries to compensate in the direction that would normalize pH. A metabolic problem gets a respiratory response and vice versa, but compensation is never so strong that it fully normalizes pH, that would mean you're looking at two separate primary disorders, not one compensated one.
| Pattern | pH | pCO₂ | HCO₃⁻ |
|---|---|---|---|
| Metabolic acidosis with respiratory compensation | ↓ | ↓ | ↓ |
| Metabolic alkalosis with respiratory compensation | ↑ | ↑ | ↑ |
| Respiratory acidosis with metabolic compensation | ↓ / near-normal | ↑ | ↑ |
| Respiratory alkalosis with metabolic compensation | ↑ / near-normal | ↓ | ↓ |
Case 1.42-year-old with persistent vomiting from gastroenteritis. pH 7.52, pCO₂ 4.8, HCO₃ 32, BE +5. pH is up, HCO₃ is up, pCO₂ is normal → metabolic alkalosis, uncompensated. Makes sense: vomiting loses stomach acid (HCl), so bicarbonate accumulates.
Case 2.27-year-old found down at home. pH 7.23, pCO₂ 3.7, HCO₃ 14, BE −15. pH is down, HCO₃ is way down, pCO₂ is also pulled down → metabolic acidosis with respiratory compensation. The lungs are blowing off CO₂ (hyperventilating) to try to drag the pH back up, but they can't fully correct it.
Case 3.64-year-old with COPD, worsening dyspnea and cough for 4 days. pH 7.26, pCO₂ 8.2, HCO₃ 28, BE +4. pH is down, pCO₂ is way up, HCO₃ is also up but not enough to normalize pH → respiratory acidosis with metabolic compensation. This is the classic decompensated COPD picture: chronic CO₂ retention with the kidneys already running a raised bicarbonate as baseline compensation, now overwhelmed by an acute flare.
1) Look at pH, decide acidosis or alkalosis. 2) Look at which of pCO₂/HCO₃ moved in the samedirection as the primary disturbance would predict, that's your primary problem. 3) Look at whether the other value moved to partially correct pH, that's compensation. Never skip straight to "is this compensated" before you've named the primary disorder.
These show up constantly, both on exams and in real dosing decisions, because raw lab values and raw body weight are frequently misleading without adjustment.
| Calculation | Formula | Use it when |
|---|---|---|
| Corrected calcium | Cacorr= [(4 − albumin) × 0.8] + Cauncorr | Albumin is abnormal (especially low); total calcium alone will mislead you |
| Ideal body weight (IBW) | 50 kg (male) or 45.5 kg (female) + 2.3 kg per inch over 5 ft | Baseline for renal dosing and weight-based calculations |
| Adjusted body weight (AdjBW) | AdjBW = IBW + 0.4 × (ABW − IBW) | Actual body weight is >30% above IBW |
Actual body weight < IBW → use actual body weight.A malnourished or frail patient's real muscle mass is lower than their ideal weight would suggest, so ideal weight would overestimate their renal function. Actual body weight within 30% of IBW → use actual body weight.Actual body weight >30% above IBW (obesity) → use adjusted body weight,because pure fat mass doesn't generate creatinine the way muscle does, so plugging in the full actual weight would overestimate CrCl. There's real variability in practice on exactly which weight clinicians choose, so when in doubt, show your reasoning.
Why the calcium correction matters practically:a patient with albumin of 2.6 g/dL and a measured total calcium of 8.1 mg/dL looks hypocalcemic at first glance. Correcting it: Cacorr= [(4 − 2.6) × 0.8] + 8.1 = (1.4 × 0.8) + 8.1 = 1.12 + 8.1 = 9.22 mg/dL, which is actually normal. Skipping the correction here would have you chasing a calcium abnormality that doesn't exist.
| Parameter | When to check | Watching for |
|---|---|---|
| BMP (Na, K, Cl, HCO₃, BUN, SCr, glucose) | Baseline, then daily in acutely ill or hospitalized patients; periodically in stable outpatients on RAAS agents or diuretics | Electrolyte drift, renal trend |
| SCr trend (not a single value) | Serially, especially after starting nephrotoxins, contrast, or RAAS blockade | AKI onset; remember SCr lags the true GFR change |
| Magnesium | Whenever potassium replacement isn't working | Concurrent hypomagnesemia driving renal K wasting |
| Corrected calcium | Any time albumin is abnormal | Masked hypo/hypercalcemia |
| CBC | Baseline and periodically on marrow-suppressive or hepatotoxic drugs | Cytopenias, infection, bleeding risk |
| LFTs | Baseline and periodically on hepatotoxic drugs | Transaminases climbing into the 1000s = drug-induced injury until proven otherwise |
| ABG | Acute respiratory or metabolic decompensation | Primary disorder plus degree of compensation |
| FENa/FEUrea | Oliguric AKI workup, before or without diuretic exposure | Prerenal vs intrinsic localization |