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 has started parenteral nutrition (PN) through a central line. Which test is monitored most frequently in the first days?

  1. ABlood glucose
  2. BHemoglobin A1c
  3. CSerum lipase
  4. DThyroid hormones
2.Lab values & electrolytesHard

Which pattern of results fits SIADH?

  1. ALow serum sodium, low serum osmolality and concentrated urine
  2. BLow serum sodium, high serum osmolality and dilute urine
  3. CHigh serum sodium, high serum osmolality and very dilute urine
  4. DNormal sodium with very dilute urine
3.Arterial blood gas interpretationMedium
pH7.357.457.29PaCO₂mmHg354532HCO₃⁻mmol/L222615uncompensatedpartially compensatedfully compensated

A practice arterial blood gas shows pH 7.29, PaCO₂ 32 mmHg (4.3 kPa) and HCO₃⁻ 15 mmol/L. The primary disorder is metabolic acidosis. How far is it compensated? Click your answer.

pH 7.29PaCO₂ 32 mmHg (4.3 kPa)HCO₃⁻ 15 mmol/L
  1. 1uncompensated
  2. 2partially compensated
  3. 3fully compensated
4.Arterial blood gas interpretationHard

A practice gas shows a metabolic alkalosis with HCO₃⁻ 32 mmol/L. Use expected PaCO₂ = 0.7·HCO₃⁻ + 21 to find the expected PaCO₂.

HCO₃⁻ 32 mmol/L
PaCO₂ = mmHg
5.Clinical pharmacokineticsMedium
10100t (h)C (mg/L)1 h30 mg/L9 h13.6 mg/L

Practice exercise (made-up values, not clinical guidance). After an IV dose of vancomycin, the level is 30 mg/L at 1 h and 13.6 mg/L at 9 h. Assume first-order elimination. Find the half-life t½.

C1 = 30 mg/L (1 h)C2 = 13.6 mg/L (9 h)
t½ = h
6.Clinical pharmacokineticsHard
VCss,avg​ = 15 mg/LD = ? mg · q8hCL = 0.035 L/kg/h93 kg · S = 0.8 · F = 1

Practice exercise (made-up values, not clinical guidance). A 93 kg patient takes oral aminophylline (S = 0.8, F = 1) every 8 h. The theophylline clearance is 0.035 L/kg/h. What dose D in mg gives an average steady-state level of 15 mg/L?

CL = 0.035 L/kg/hτ = 8 hS = 0.8F = 193 kgCss,avg = 15 mg/L
D = mg
7.Clinical nursingMedium

Which group of features is typical of nephrotic syndrome?

  1. ABlood in the urine, high blood pressure and reduced urine output only
  2. BHeavy proteinuria, low serum albumin and oedema
  3. CSevere loin pain and fever
  4. DPolyuria, thirst and weight loss
8.Clinical nursingHard

A patient with suspected meningitis has fever, neck stiffness and a non-blanching rash. What is the priority?

  1. AWait for the lumbar puncture result before starting any treatment
  2. BDim the lights, encourage rest and recheck observations in four hours
  3. CUrgent review and immediate antibiotics, without waiting for tests
  4. DGive paracetamol for the fever and review the patient tomorrow

3Solutions

1.Lab values & electrolytes

Answer: A · Blood glucose

Why this is the answer

PN contains concentrated dextrose, so hyperglycemia is common, especially at the start; glucose is checked frequently and insulin may be needed. Electrolytes, including phosphate, magnesium and potassium, are also monitored for refeeding syndrome.

Answer

A · Blood glucose
2.Lab values & electrolytes

Answer: A · Low serum sodium, low serum osmolality and concentrated urine

Why this is the answer

Excess antidiuretic hormone causes water retention, diluting the blood (low sodium and osmolality), while the urine is inappropriately concentrated. High sodium with very dilute urine is the pattern of diabetes insipidus.Checked against: Syndrome of inappropriate antidiuretic hormone secretion · MedlinePlus (NIH) (2025)

Answer

A · Low serum sodium, low serum osmolality and concentrated urine
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 7.29: acidaemia (below 7.35)
PaCO₂ 32 mmHg: low (35–45)
HCO₃⁻ 15 mmol/L: low (22–26)

Compensation

partially compensated

Answer

partially compensated
4.Arterial blood gas interpretation

Answer: PaCO₂ = 43.4 mmHg

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

0.7·32 + 21 = 43.4 mmHg

Answer

PaCO₂ = 43.4 mmHg

The rise has a limit: PaCO₂ rarely goes above about 55 mmHg from compensation alone.

5.Clinical pharmacokinetics

Answer: t½ = 7.009 h

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.

Elimination rate constant

k = ln(30/13.6)/8 = 0.7911/8 = 0.09889 h⁻¹

Half-life

t½ = 0.693/0.09889 = 7.009 h

Answer

t½ = 7.009 h
6.Clinical pharmacokinetics

Answer: D = 488.3 mg

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 = 0.035·93 = 3.255 L/h

Dose

D = Css,avg·CL·τ/(S·F) = 15·3.255·8/(0.8·1) = 488.3 mg

Answer

D = 488.3 mg
7.Clinical nursing

Answer: B · Heavy proteinuria, low serum albumin and oedema

Why this is the answer

In nephrotic syndrome, damaged glomeruli leak protein, so albumin falls and fluid moves into tissues, causing oedema. Cholesterol is often raised too.

Answer

B · Heavy proteinuria, low serum albumin and oedema
8.Clinical nursing

Answer: C · Urgent review and immediate antibiotics, without waiting for tests

Why this is the answer

Bacterial meningitis and meningococcal sepsis progress rapidly. Antibiotics should not be delayed for tests such as lumbar puncture.

Answer

C · Urgent review and immediate antibiotics, without waiting for tests

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