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

When several samples are drawn at the same venipuncture, which tubes or bottles are filled first?

  1. ABlood culture bottles
  2. BFluoride (glucose) tubes
  3. CCoagulation (citrate) tubes
  4. DEDTA (complete blood count) tubes
2.Lab values & electrolytesHard

A 68-year-old man has fatigue, a hemoglobin of 9.8 g/dL (98 g/L), an MCV of 72 fL and a low ferritin. He eats a normal diet. What is the most important next step?

  1. ALook for a source of blood loss in the gut
  2. BGive iron and do no further tests at all
  3. CStart vitamin B12 injections
  4. DRepeat the blood count in one year
3.Arterial blood gas interpretationMedium

A practice gas shows a metabolic acidosis with HCO₃⁻ 8 mmol/L. Use Winter’s formula to find the expected PaCO₂ (the centre of the range) in mmHg.

HCO₃⁻ 8 mmol/L
PaCO₂ = mmHg
4.Arterial blood gas interpretationHard
pH7.357.457.20PaCO₂mmHg354521HCO₃⁻mmol/L22268added normal-gap acidosispure high-gap acidosisadded metabolic alkalosis

A practice gas shows pH 7.20, PaCO₂ 21 mmHg (2.8 kPa) and HCO₃⁻ 8 mmol/L, with Na⁺ 141 and Cl⁻ 99 mmol/L. Use the anion gap and the delta ratio to click the best interpretation.

pH 7.20PaCO₂ 21 mmHg (2.8 kPa)HCO₃⁻ 8 mmol/LNa⁺ 141 mmol/LCl⁻ 99 mmol/L
  1. 1added normal-gap acidosis
  2. 2pure high-gap acidosis
  3. 3added metabolic alkalosis
5.Clinical pharmacokineticsMedium
Vd = 7 L/kgC = 1 mcg/LLD = ? mcg50 kg · S = 1 · F = 0.7

Practice exercise (made-up values, not clinical guidance). A 50 kg patient is to be loaded with digoxin tablets to reach a level of 1 mcg/L. The volume of distribution is 7 L/kg and the tablets have a bioavailability F = 0.7. Find the total oral loading dose LD in mcg.

50 kgVd = 7 L/kgC = 1 mcg/LF = 0.7
LD = mcg
6.Clinical pharmacokineticsHard
66 kg63 · manSCr 1.9 mg/dLfe​ = 0.6D = ? mg

Practice exercise (made-up values, not clinical guidance). A 63-year-old man weighs 66 kg and has a serum creatinine of 1.9 mg/dL (168 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 ciprofloxacin dose is 500 mg for a creatinine clearance of 120 mL/min, and a fraction fe = 0.6 of the drug is excreted unchanged by the kidneys. Using the Dettli dose fraction Q = 1 − fe·(1 − CrCl/normal CrCl), find the adjusted dose D. Round to the nearest whole number.

63 years66 kgmanSCr = 1.9 mg/dL500 mgfe = 0.6CrCl normal 120 mL/min
D = mg
7.Clinical nursingMedium

An unconscious patient with diabetes has a blood glucose of 2.1 mmol/L and no IV access. What can be given?

  1. AOral fruit juice
  2. BSubcutaneous insulin
  3. CGlucose tablets by mouth
  4. DIntramuscular glucagon
8.Clinical nursingHard

A patient after abdominal surgery has not passed urine for eight hours, has lower abdominal discomfort and a palpable bladder. What is the most likely problem?

  1. AAcute kidney injury with no urine production
  2. BUrinary tract infection without retention
  3. CAcute urinary retention
  4. DDehydration only

3Solutions

1.Lab values & electrolytes

Answer: A · Blood culture bottles

Why this is the answer

Blood cultures are drawn first to reduce contamination. A standard order of draw then follows (citrate, serum, heparin, EDTA, fluoride), which prevents additives being carried from one tube into the next and distorting results.

Answer

A · Blood culture bottles
2.Lab values & electrolytes

Answer: A · Look for a source of blood loss in the gut

Why this is the answer

Iron deficiency in an older man or a postmenopausal woman suggests chronic blood loss, often from the gastrointestinal tract, including colorectal cancer. Iron is started, but endoscopy or colonoscopy is needed to find the cause.

Answer

A · Look for a source of blood loss in the gut
3.Arterial blood gas interpretation

Answer: PaCO₂ = 20 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.

Winter’s formula

PaCO₂ = 1.5·8 + 8 = 20 mmHg
range: 18–22 mmHg

Answer

PaCO₂ = 20 mmHg
4.Arterial blood gas interpretation

Answer: pure high-gap acidosis

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.

Anion gap

AG = 141 − (99 + 8) = 34

Delta ratio

(34 − 12)/(24 − 8) = 1.375
pure high-gap acidosis

Answer

pure high-gap acidosis
5.Clinical pharmacokinetics

Answer: LD = 500 mcg

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 = 7·50 = 350 L

Amount of active drug needed

Vd·C = 350·1 = 350 mcg

Loading dose

LD = 350/(1·0.7) = 500 mcg

Answer

LD = 500 mcg

In practice the dose is rounded to a convenient amount.

6.Clinical pharmacokinetics

Answer: D = 293 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.

Creatinine clearance (Cockcroft–Gault)

CrCl = (140 − 63)·66/(72·1.9) = 37.15 mL/min

Dose fraction (Dettli)

Q = 1 − 0.6·(1 − 37.15/120) = 0.5857

Adjusted dose

D = 0.5857·500 = 292.9 ≈ 293 mg

Answer

D = 293 mg

In practice the dose is rounded to a convenient amount and checked against the product information.

7.Clinical nursing

Answer: D · Intramuscular glucagon

Why this is the answer

In unconscious hypoglycaemia, oral treatment risks aspiration. IM glucagon releases liver glycogen; IV glucose is used if access is available.

Answer

D · Intramuscular glucagon
8.Clinical nursing

Answer: C · Acute urinary retention

Why this is the answer

Anaesthesia, opioids and pain can cause retention. A bladder scan confirms residual volume; catheterisation is often needed.

Answer

C · Acute urinary retention

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