Lab Values for Nurses: cram pack

Formula sheets, the key method steps for every topic and 8 practice problems, with full solutions at the back.

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1Formulas and methods

Lab values & electrolytes

Method

  1. Compare the value with the laboratory’s own reference range, then with the patient’s baseline and trend.
  2. Check the sample: hemolysis, a delay, or blood drawn near an IV can give false results.
  3. Link the value to its typical signs and to the drugs and conditions that cause it.
  4. Look for interactions: low potassium or magnesium increases digoxin toxicity; dehydration raises lithium.
  5. Act on the patient: assess, hold a drug if the level is unsafe, read back critical values and inform the prescriber.

Arterial blood gas interpretation

Normal ranges
pH 7.35–7.45, PaCO₂ 35–45 mmHg (4.7–6.0 kPa), HCO₃⁻ 22–26 mmol/L
ROME
Respiratory: PaCO₂ opposite to pH; Metabolic: HCO₃⁻ equal to pH
Winter’s formula
PaCO₂ = 1.5·HCO₃⁻ + 8 ± 2
Metabolic alkalosis
PaCO₂ ≈ 0.7·HCO₃⁻ + 21
Anion gap
AG = Na⁺ − (Cl⁻ + HCO₃⁻) ≈ 8–12
Albumin-corrected gap
AG + 2.5·(4.0 − alb g/dL) = AG + 0.25·(40 − alb g/L)
Delta ratio
(AG − 12)/(24 − HCO₃⁻)
P/F ratio
PaO₂/FiO₂
Alveolar gas equation
PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8
Serum osmolality
2·Na⁺ + glucose + urea (2·Na⁺ + glucose/18 + BUN/2.8)

Method

  1. Read the pH: below 7.35 is acidosis, above 7.45 alkalosis.
  2. Find the value that explains it: PaCO₂ moving opposite to the pH is respiratory, HCO₃⁻ moving with the pH is metabolic.
  3. Look at the other value: normal means uncompensated, abnormal in the compensating direction means partial compensation, and a normal pH with both abnormal means full compensation.
  4. In a metabolic acidosis, compare PaCO₂ with Winter’s formula, 1.5·HCO₃⁻ + 8 ± 2, and calculate the anion gap Na⁺ − (Cl⁻ + HCO₃⁻).
  5. Check a high anion gap with the delta ratio and, when albumin is low, with the corrected gap.

Clinical pharmacokinetics

Rate constant and half-life
k = ln 2/t½ = 0.693/t½
First-order decay
C = C₀·e−kt = C₀·(1/2)t/t½
Two levels
k = ln(C₁/C₂)/Δt
Volume and clearance
Vd = dose/C₀, CL = k·Vd
Loading dose
LD = Vd·C/(S·F)
Maintenance rate
R = CL·Css/(S·F)
Average steady state
Css,avg = S·F·D/(CL·τ)
Steady state
1 − (1/2)ⁿ after n half-lives; ≈ 97% after 5
Infusion
C = (R/CL)(1 − e−kt)
Interval and dose from peak and trough
τ = ln(Cpeak/Ctrough)/k, D = Vd·(Cpeak − Ctrough)
Cockcroft–Gault
CrCl = (140 − age) × kg/(72 × SCr mg/dL), × 0.85 for women
Renal adjustment
dose × CrCl/normal; Dettli Q = 1 − fe·(1 − CrCl/normal)

Method

  1. Convert the half-life to a rate constant: k = 0.693/t½.
  2. Turn per-kg values into totals: Vd (L) = L/kg × weight, CL (L/h) = L/kg/h × weight.
  3. Pick the relation: C = C₀·e−kt, LD = Vd·C/(S·F), maintenance rate = CL·Css/(S·F), τ = ln(Cpeak/Ctrough)/k.
  4. For kidney function, estimate CrCl with Cockcroft–Gault first, then scale the dose or the interval.
  5. Check the result: levels fall by half every half-life, and steady state takes about five half-lives.

2Practice problems

1.Lab values & electrolytesMedium

A patient with suspected pulmonary TB needs sputum samples for acid-fast bacilli. Which instruction is correct?

  1. AGive several samples of nasal mucus blown into a tissue
  2. BGive one sample of saliva collected from the mouth at any time of day
  3. CGive one sample after brushing with mouthwash so it is clean
  4. DGive several deep-cough samples, often first thing in the morning
2.Lab values & electrolytesHard

At the start of a shift, the nurse reviews four new results. Which patient needs attention first?

  1. APotassium 5.2 mEq/L in a patient with stable chronic kidney disease
  2. BGlucose 180 mg/dL (10 mmol/L) after a meal in a patient with diabetes
  3. CSodium 119 mEq/L in a patient with new confusion and vomiting
  4. DHemoglobin 11.0 g/dL (110 g/L) in a patient with rheumatoid arthritis
3.Arterial blood gas interpretationMedium
pH7.357.457.24PaCO₂mmHg354524HCO₃⁻mmol/L222610uncompensatedpartially compensatedfully compensated

A practice arterial blood gas shows pH 7.24, PaCO₂ 24 mmHg (3.2 kPa) and HCO₃⁻ 10 mmol/L. The primary disorder is metabolic acidosis. How far is it compensated? Click your answer.

pH 7.24PaCO₂ 24 mmHg (3.2 kPa)HCO₃⁻ 10 mmol/L
  1. 1uncompensated
  2. 2partially compensated
  3. 3fully compensated
4.Arterial blood gas interpretationHard

At sea level, a practice gas taken on 40% oxygen shows PaO₂ 80 mmHg (10.7 kPa) and PaCO₂ 32 mmHg (4.3 kPa). Use PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 to find the A–a gradient.

FiO₂ 40%PaO₂ 80 mmHg (10.7 kPa)PaCO₂ 32 mmHg (4.3 kPa)
A–a gradient = mmHg
5.Clinical pharmacokineticsMedium
Vd = 0.5 L/kgC = 12 mg/LLD = ? mg60 kg · S = 0.8 · F = 1

Practice exercise (made-up values, not clinical guidance). A 60 kg patient is to be loaded intravenously with aminophylline to reach a theophylline level of 12 mg/L. The volume of distribution is 0.5 L/kg and the salt factor is S = 0.8. Find the loading dose LD of aminophylline in mg.

60 kgVd = 0.5 L/kgC = 12 mg/LS = 0.8
LD = mg
6.Clinical pharmacokineticsHard
Vd = 7 L/kg0.6 → 1 mcg/LLD = ? mcg75 kg · S = 1 · F = 0.7

Practice exercise (made-up values, not clinical guidance). A 75 kg patient has a digoxin level of 0.6 mcg/L; the target is 1 mcg/L. The volume of distribution is 7 L/kg and the tablets have F = 0.7. What extra loading dose LD raises the level to the target?

75 kgVd = 7 L/kgC = 0.6 → 1 mcg/LF = 0.7
LD = mcg
7.Clinical nursingMedium

Why is serum lactate measured in a patient with suspected sepsis?

  1. AA raised lactate indicates poor tissue perfusion and higher risk
  2. BIt confirms which antibiotic to give
  3. CIt shows whether the patient has a urinary tract infection
  4. DIt measures the patient's blood glucose
8.Clinical nursingHard

Why should chronic hypernatraemia be corrected gradually?

  1. ASlow correction is only needed in children
  2. BFast correction causes severe hyperkalaemia
  3. CFast correction causes osmotic demyelination from swelling of the kidneys
  4. DLowering the sodium too fast can cause cerebral oedema

3Solutions

1.Lab values & electrolytes

Answer: D · Give several deep-cough samples, often first thing in the morning

Why this is the answer

Usually a series of three sputum samples is collected, at least one in the early morning, when secretions have pooled overnight. The patient rinses with water (not mouthwash), takes deep breaths and coughs deeply to bring up sputum, not saliva. Airborne precautions are used.Checked against: Sputum Culture · MedlinePlus (NIH) (2024); Acid-Fast Bacillus (AFB) Tests · MedlinePlus (NIH) (2025)

Answer

D · Give several deep-cough samples, often first thing in the morning
2.Lab values & electrolytes

Answer: C · Sodium 119 mEq/L in a patient with new confusion and vomiting

Why this is the answer

Severe hyponatremia with confusion and vomiting suggests cerebral edema and can progress to seizures and coma. The other results are mildly abnormal and expected for those conditions, so they are less urgent.Checked against: Low blood sodium - Medical Encyclopedia · MedlinePlus (NIH) (2025); Sodium Blood Test · MedlinePlus (NIH) (2024)

Answer

C · Sodium 119 mEq/L in a patient with new confusion and vomiting
3.Arterial blood gas interpretation

Answer: partially compensated

Reading a blood gas (ROME): first the pH: below 7.35 is acidaemia, above 7.45 alkalaemia. Then find the value that explains it. PaCO₂ (normal 35–45 mmHg, 4.7–6.0 kPa) is the respiratory part and moves Opposite to the pH in a respiratory disorder; HCO₃⁻ (normal 22–26 mmol/L) is the metabolic part and moves in the same direction as the pH (Respiratory Opposite, Metabolic Equal). The other value shows the compensation: normal means uncompensated; abnormal in the direction that pulls the pH back while the pH is still abnormal means partially compensated; a normal pH with both values abnormal means fully compensated. Ranges differ slightly between laboratories; these are practice values.

ROME, value by value

pH 7.24: acidaemia (below 7.35)
PaCO₂ 24 mmHg: low (35–45)
HCO₃⁻ 10 mmol/L: low (22–26)

Compensation

partially compensated

Answer

partially compensated
4.Arterial blood gas interpretation

Answer: A–a gradient = 165.2 mmHg

Derived values: calculated serum osmolality is 2·Na⁺ + glucose + urea with all values in mmol/L (2·Na⁺ + glucose/18 + BUN/2.8 with glucose and BUN in mg/dL); normal is about 275–295 mOsm/kg. The P/F ratio is PaO₂ divided by FiO₂ as a fraction. The alveolar oxygen pressure is PAO₂ = FiO₂·(760 − 47) − PaCO₂/0.8 mmHg at sea level, and the A–a gradient is PAO₂ − PaO₂.

Alveolar oxygen

PAO₂ = 0.4·713 − 32/0.8 = 285.2 − 40 = 245.2 mmHg

Gradient

A–a = 245.2 − 80 = 165.2 mmHg

Answer

A–a gradient = 165.2 mmHg
5.Clinical pharmacokinetics

Answer: LD = 450 mg

Clearance and volume: at steady state the dosing rate equals the elimination rate, so the maintenance rate = CL × Css. A loading dose is LD = Vd × C/(S × F), with S the salt factor (fraction of active drug in the salt) and F the bioavailability (1 for IV). From two levels, k = ln(C₁/C₂)/Δt; for an IV bolus Vd = dose/C₀ and CL = k × Vd. The numbers are for practice only; at the bedside always follow local protocols and have high-risk doses double-checked.

Volume of distribution

Vd = 0.5·60 = 30 L

Amount of active drug needed

Vd·C = 30·12 = 360 mg

Loading dose

LD = 360/(0.8·1) = 450 mg

Answer

LD = 450 mg

In practice the dose is rounded to a convenient amount.

6.Clinical pharmacokinetics

Answer: LD = 300 mcg

Designing a regimen: with rapid IV doses the level falls from peak to trough during the interval, so τ = ln(Cpeak/Ctrough)/k and the dose that restores the peak is Vd × (Cpeak − Ctrough). The average steady-state level is Css,avg = S·F·dose/(CL·τ). An infusion rises as C = (R/CL)(1 − e−kt). For a drug cleared by the kidneys, the dose is scaled with the creatinine clearance. The numbers are for practice only; at the bedside always follow local protocols and have high-risk doses double-checked.

Volume of distribution

Vd = 7·75 = 525 L

Rise needed

1 − 0.6 = 0.4 mcg/L

Loading dose

LD = 525·0.4/(1·0.7) = 300 mcg

Answer

LD = 300 mcg

In practice the dose is rounded to a convenient amount.

7.Clinical nursing

Answer: A · A raised lactate indicates poor tissue perfusion and higher risk

Why this is the answer

Lactate rises when tissues are poorly perfused. A high lactate in sepsis signals greater severity and is used to guide fluid resuscitation.

Answer

A · A raised lactate indicates poor tissue perfusion and higher risk
8.Clinical nursing

Answer: D · Lowering the sodium too fast can cause cerebral oedema

Why this is the answer

In chronic hypernatraemia, brain cells adapt by holding extra solutes. If sodium falls too quickly, water moves into brain cells and causes cerebral oedema, so correction is controlled.

Answer

D · Lowering the sodium too fast can cause cerebral oedema

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