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 on a heparin infusion has an aPTT below the target range on the protocol. What should the nurse do?

  1. ASwitch the infusion to oral warfarin without a prescription
  2. BAdjust the rate as the protocol directs and recheck as scheduled
  3. CContinue the same rate and recheck the aPTT in a few days
  4. DStop the infusion and give protamine sulfate as quickly as possible
2.Lab values & electrolytesHard

Two days after bowel surgery, a patient with nasogastric suction has a distended abdomen, absent bowel sounds and muscle weakness. The potassium is 2.8 mEq/L. Which interpretation is best?

  1. AThe low potassium is due to the patient drinking too much water
  2. BThe potassium is normal for the second day after surgery
  3. CHyperkalemia from tissue damage is causing the bowel to stop
  4. DHypokalemia from gastric losses is contributing to paralytic ileus
3.Arterial blood gas interpretationMedium

Practice values: Na⁺ 135 mmol/L, glucose 4.3 mmol/L and urea 2.6 mmol/L. Calculate the serum osmolality Osm = 2·Na⁺ + glucose + urea to the nearest whole number.

Na⁺ 135 mmol/Lglucose 4.3 mmol/Lurea 2.6 mmol/L
Osm = mOsm/kg
4.Arterial blood gas interpretationHard
pH7.357.457.31PaCO₂mmHg354527HCO₃⁻mmol/L222613appropriate compensationadded respiratory acidosisadded respiratory alkalosis

A practice gas shows a metabolic acidosis: pH 7.31, PaCO₂ 27 mmHg (3.6 kPa), HCO₃⁻ 13 mmol/L. Compare the measured PaCO₂ with Winter’s formula. Click your interpretation.

pH 7.31PaCO₂ 27 mmHg (3.6 kPa)HCO₃⁻ 13 mmol/L
  1. 1appropriate compensation
  2. 2added respiratory acidosis
  3. 3added respiratory alkalosis
5.Clinical pharmacokineticsMedium
Vd = 0.5 L/kgC = 12 mg/LLD = ? mg99 kg · S = 0.8 · F = 1

Practice exercise (made-up values, not clinical guidance). A 99 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.

99 kgVd = 0.5 L/kgC = 12 mg/LS = 0.8
LD = mg
6.Clinical pharmacokineticsHard
VCss,avg​ = ?200 mg · q8hCL = 0.03 L/kg/h54 kg · S = 1 · F = 1

Practice exercise (made-up values, not clinical guidance). A 54 kg patient takes oral theophylline (F = 1) every 8 h. The clearance is 0.03 L/kg/h. Each dose is 200 mg of theophylline. Find the average steady-state level Css,avg.

CL = 0.03 L/kg/hτ = 8 hS = 1F = 154 kgD = 200 mg
Css,avg = mg/L
7.Clinical nursingMedium

Which group of symptoms suggests hypercalcaemia?

  1. ATingling lips, muscle cramps and spasms of the hands and feet
  2. BSweating, tremor and sudden hunger
  3. CThirst, constipation, confusion and abdominal pain
  4. DWeight gain, feeling cold and a slow pulse
8.Clinical nursingHard

How should pain be assessed in a patient with advanced dementia who cannot describe it?

  1. AWith an observational pain tool that scores facial expression, behaviour and body language
  2. BBy checking the blood pressure only
  3. CBy asking the patient to rate their pain on a numerical scale from 0 to 10
  4. DBy assuming the patient has no pain unless they are crying out or calling for help

3Solutions

1.Lab values & electrolytes

Answer: B · Adjust the rate as the protocol directs and recheck as scheduled

Why this is the answer

A subtherapeutic aPTT means the patient is not fully anticoagulated and is at risk of clot extension. Weight-based heparin protocols specify a bolus and/or rate increase and when to recheck. The nurse follows the protocol and documents the change.Checked against: Partial Thromboplastin Time (PTT) Test · MedlinePlus (NIH) (2026); Partial thromboplastin time (PTT) - Medical Encyclopedia · MedlinePlus (NIH) (2025)

Answer

B · Adjust the rate as the protocol directs and recheck as scheduled
2.Lab values & electrolytes

Answer: D · Hypokalemia from gastric losses is contributing to paralytic ileus

Why this is the answer

Gastric suction removes potassium and causes metabolic alkalosis, which shifts more potassium into cells. Low potassium reduces smooth muscle contraction and contributes to ileus and weakness. The provider is informed and potassium is replaced as prescribed.Checked against: Hypokalemia · StatPearls (NCBI Bookshelf) (2025)

Answer

D · Hypokalemia from gastric losses is contributing to paralytic ileus
3.Arterial blood gas interpretation

Answer: Osm = 277 mOsm/kg

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.

Calculated osmolality

Osm = 2·135 + 4.3 + 2.6 = 276.9 ≈ 277 mOsm/kg

Answer

Osm = 277 mOsm/kg
4.Arterial blood gas interpretation

Answer: appropriate compensation

Expected compensation: in metabolic acidosis Winter’s formula predicts PaCO₂ = 1.5·HCO₃⁻ + 8 ± 2 mmHg; a measured value above that range points to an additional respiratory acidosis, below it to an additional respiratory alkalosis. The anion gap AG = Na⁺ − (Cl⁻ + HCO₃⁻), normally about 8–12 mmol/L, separates high-gap from normal-gap metabolic acidosis; low albumin hides a high gap (add 2.5 per g/dL that albumin is below 4.0). The delta ratio (AG − 12)/(24 − HCO₃⁻) between about 0.8 and 2 fits a pure high-gap acidosis (some texts use 1 instead of 0.8), below that an added normal-gap acidosis, and above 2 an added metabolic alkalosis.

Expected PaCO₂

1.5·13 + 8 = 27.5 → 25.5–29.5 mmHg

Compare

measured: 27 mmHg → appropriate compensation

Answer

appropriate compensation
5.Clinical pharmacokinetics

Answer: LD = 742.5 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 = 0.5·99 = 49.5 L

Amount of active drug needed

Vd·C = 49.5·12 = 594 mg

Loading dose

LD = 594/(0.8·1) = 742.5 mg

Answer

LD = 742.5 mg

In practice the dose is rounded to a convenient amount.

6.Clinical pharmacokinetics

Answer: Css,avg = 15.43 mg/L

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.

Clearance

CL = 0.03·54 = 1.62 L/h

Average steady-state level

Css,avg = S·F·D/(CL·τ) = 1·1·200/(1.62·8) = 15.43 mg/L

Answer

Css,avg = 15.43 mg/L
7.Clinical nursing

Answer: C · Thirst, constipation, confusion and abdominal pain

Why this is the answer

High calcium causes polyuria and thirst, constipation, abdominal pain, kidney stones and confusion. Tingling and cramps suggest low calcium instead.

Answer

C · Thirst, constipation, confusion and abdominal pain
8.Clinical nursing

Answer: A · With an observational pain tool that scores facial expression, behaviour and body language

Why this is the answer

Observational tools such as PAINAD or the Abbey Pain Scale help assess pain in people who cannot self-report. Family and carers can describe changes in usual behaviour.

Answer

A · With an observational pain tool that scores facial expression, behaviour and body language

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