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 55-year-old has a positive fecal immunochemical test (FIT) during routine screening. What is the usual next step?

  1. ARepeat the test in 10 years
  2. BStart an iron supplement only
  3. CReferral for colonoscopy
  4. DNo action if there are no symptoms
2.Lab values & electrolytesHard

A woman with preeclampsia is receiving IV magnesium sulfate. Her urine output has fallen and her creatinine is rising. Why is this a concern?

  1. AMagnesium is excreted by the kidneys, so it can build up to toxic levels
  2. BPoor kidney function makes the magnesium level fall quickly
  3. CMagnesium is broken down by the liver, so it will stop working soon
  4. DMagnesium causes potassium loss, so the kidneys release more fluid
3.Arterial blood gas interpretationMedium
pH7.357.457.59PaCO₂mmHg354542HCO₃⁻mmol/L222639uncompensatedpartially compensatedfully compensated

A practice arterial blood gas shows pH 7.59, PaCO₂ 42 mmHg (5.6 kPa) and HCO₃⁻ 39 mmol/L. The primary disorder is metabolic alkalosis. How far is it compensated? Click your answer.

pH 7.59PaCO₂ 42 mmHg (5.6 kPa)HCO₃⁻ 39 mmol/L
  1. 1uncompensated
  2. 2partially compensated
  3. 3fully compensated
4.Arterial blood gas interpretationHard
pH7.357.457.21PaCO₂mmHg354570HCO₃⁻mmol/L222627acutechronicacute on chronic

A practice gas shows a respiratory acidosis: pH 7.21, PaCO₂ 70 mmHg (9.3 kPa), HCO₃⁻ 27 mmol/L. Use the rules ‘HCO₃⁻ rises 1 mmol/L per 10 mmHg (acute) or 3.5 mmol/L per 10 mmHg (chronic)’ to classify it. Click your answer.

pH 7.21PaCO₂ 70 mmHg (9.3 kPa)HCO₃⁻ 27 mmol/L
  1. 1acute
  2. 2chronic
  3. 3acute on chronic
5.Clinical pharmacokineticsMedium
VCss​ = 2 mg/LR = ? mg/minCL = 750 mL/min

Practice exercise (made-up values, not clinical guidance). A patient has a lidocaine clearance of 750 mL/min. What infusion rate R in mg/min keeps the steady-state level at 2 mg/L?

CL = 750 mL/minCss = 2 mg/L
R = mg/min
6.Clinical pharmacokineticsHard
69 kg40 · manSCr 2.5 mg/dLCrCl → τ = ? h

Practice exercise (made-up values, not clinical guidance). A 40-year-old man weighs 69 kg and has a serum creatinine of 2.5 mg/dL (221 µmol/L). Estimate the creatinine clearance with Cockcroft–Gault: CrCl (mL/min) = (140 − age) × weight (kg)/(72 × SCr in mg/dL), × 0.85 for women. The usual gentamicin regimen is 80 mg every 8 h for a creatinine clearance of 100 mL/min. Keeping the dose, the interval is lengthened in proportion to the fall in clearance. Find the new dosing interval τ. Round to one decimal place.

40 years69 kgmanSCr = 2.5 mg/dL80 mg q8hCrCl normal 100 mL/min
τ = h
7.Clinical nursingMedium

A nurse suspects that an older patient is being financially abused by a relative. What should the nurse do?

  1. ARaise the concern through the local safeguarding (adult protection) procedure
  2. BKeep it secret because it is a family matter
  3. CWait until the patient has been discharged
  4. DConfront the relative directly at the bedside about it
8.Clinical nursingHard

Which type of drug is commonly used to treat neuropathic pain?

  1. ASome anticonvulsants or antidepressants, such as gabapentin
  2. BProton pump inhibitors such as omeprazole
  3. CAntibiotics such as amoxicillin, given as a long course
  4. DAntihistamines such as cetirizine

3Solutions

1.Lab values & electrolytes

Answer: C · Referral for colonoscopy

Why this is the answer

A positive FIT means hidden blood in the stool, which may come from a polyp, cancer or other lesion, so a colonoscopy is needed to find the source. A positive screening test is never ignored, even without symptoms.Checked against: Fecal Occult Blood Test (FOBT) · MedlinePlus (NIH) (2024)

Answer

C · Referral for colonoscopy
2.Lab values & electrolytes

Answer: A · Magnesium is excreted by the kidneys, so it can build up to toxic levels

Why this is the answer

Magnesium is cleared by the kidneys, so falling urine output and rising creatinine raise the risk of toxicity. The nurse checks deep tendon reflexes, respiratory rate and level of consciousness, reports the findings and keeps calcium gluconate available as the antidote.Checked against: Magnesium Toxicity (StatPearls) · StatPearls (NCBI Bookshelf) (2026)

Answer

A · Magnesium is excreted by the kidneys, so it can build up to toxic levels
3.Arterial blood gas interpretation

Answer: uncompensated

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 7.59: alkalaemia (above 7.45)
PaCO₂ 42 mmHg: normal (35–45)
HCO₃⁻ 39 mmol/L: high (22–26)

Compensation

Clinically, also check the expected compensation: for a metabolic alkalosis PaCO₂ ≈ 0.7·HCO₃⁻ + 21 = 46.3–50.3 mmHg, so the measured 42 mmHg points to an additional respiratory alkalosis (a mixed disorder). The answer above is the ROME exam classification.

uncompensated

Answer

uncompensated
4.Arterial blood gas interpretation

Answer: acute

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 HCO₃⁻

acute: 24 + 1·3 = 27 mmol/L
chronic: 24 + 3.5·3 = 34.5 mmol/L

Compare

measured: 27 mmol/L → acute

Answer

acute
5.Clinical pharmacokinetics

Answer: R = 1.5 mg/min

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.

Clearance in L/min

750 mL/min = 0.75 L/min

Infusion rate

R = 0.75·2 = 1.5 mg/min

Answer

R = 1.5 mg/min
6.Clinical pharmacokinetics

Answer: τ = 20.9 h

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.

Creatinine clearance (Cockcroft–Gault)

CrCl = (140 − 40)·69/(72·2.5) = 38.33 mL/min

New interval

τ = 8·100/38.33 = 20.87 ≈ 20.9 h

Answer

τ = 20.9 h

In practice the interval is rounded to a convenient value such as 12, 24, 36 or 48 h.

7.Clinical nursing

Answer: A · Raise the concern through the local safeguarding (adult protection) procedure

Why this is the answer

Financial abuse is a form of adult abuse. Nurses report concerns through safeguarding procedures, document facts accurately and involve the patient as appropriate.

Answer

A · Raise the concern through the local safeguarding (adult protection) procedure
8.Clinical nursing

Answer: A · Some anticonvulsants or antidepressants, such as gabapentin

Why this is the answer

Neuropathic pain responds poorly to standard analgesics. Drugs that modify nerve signalling, such as gabapentinoids and some antidepressants, are used, with doses increased gradually.

Answer

A · Some anticonvulsants or antidepressants, such as gabapentin

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