Free points on your NBME Shelf Exams and USMLE Step 1

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Free points on your NBME Shelf Exams and USMLE Step 1

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Now this isn’t the sexiest topic in medicine, but it really is an important one. So pay close attention and you’ll be breezing through the boards in no time.

As you may know, USMLE step 1 comprises 336 questions divided into 7 blocks of 48 questions. At 48 questions per block that leaves you with an average of 1.25 minutes per question (1 minute and 15 seconds).

One of the simplest ways to save time and earn more points on USMLE Step 1 and your NBME Shelf exams is to know your lab values.

Without a doubt, you will encounter many vignettes that will include lab values. You will be required to interpret these values in order to answer the question correctly. Information presented to you from the history and physical can point you in the right direction, but if lab values are given and you don’t know how to interpret them, you’re pretty much just making an educated guess. Clinicians in the real world won’t write a diagnosis without lab confirmation if they can avoid it, and on tests, you shouldn’t either.

Let me show you what I mean.

Sample vignette:

You are presented with a patient that has fever, fatigue and tender hepatomegaly. A liver profile study reveals mild elevation in levels of AST and ALT (AST: 496 IU/L and ALT: 212 IU/L). The patient has a history of drinking 80 ounces of beer daily for the past 25 years and denies any blood transfusions, tattoos and any high risk sexual activity. What is the most likely diagnosis?

Your approach:

Firstly, you notice that this patient’s AST and ALT levels are significantly higher than the upper limit of normal for AST and ALT (20 IU/L for both enzymes). Next we recall that the patient has an enlarged liver, fatigue and a fever. So we’re thinking that the patient has hepatitis. Next, you notice that the AST:ALT ratio is slightly higher than 2:1. An AST:ALT ratio of 2:1 is commonly found in alcoholic hepatitides. Based on the patient’s history, we can be quite confident that viral hepatitis is an unlikely diagnosis since our patient does not have any risk factors (blood transfusions, tattoos, or risky sexual activity). Also, the ALT would be greater than the AST in the patient’s lab work if this were the case.

On your NBME Shelf exams and USMLE Step 1 you will be given reference ranges for many different lab values. As you might expect, this is a lot of information. Before you can interpret lab data presented to you in a vignette, you have to know the reference range for that marker. If you don’t know it off hand, you have to locate it among all the other lab values available. This can be harder than you think if you’re under the gun in an exam and time’s a tickin’.

To save time, familiarize yourself the different lab sections. In that regard, the new FRED 2 – the new USMLE software program has a search function that makes this a lot easier. Lab values are subdivided into four sections:

1. Blood

2. Hematologic

3. Cerebrospinal Fluid

4. Sweat, Urine, BMI

If you’re presented with a patient who has a K+ level of 3.1 and you don’t know the reference range, knowing where to locate this value will make your life a lot easier. You should know that this value is located in section 1- Blood. The reference range for K+ is 3.5 to 5.0mEq/L by the way.

Obviously, memorizing lab data that you will have at your finger tips during an exam is a waste of time. But you should be familiar with some very commonly ordered laboratory studies.

Common lab studies you should be familiar with include:

basic metabolic profile (BMP), complete blood count (CBC), liver profile, cardiac enzymes, arterial blood gas (ABG), and lipid profile.

BMP includes: Na+, K+, Cl-, HC03-, blood urea nitrogen (BUN), creatinine, glucose

CBC includes: white blood cell count (WBC), hemoglobin (Hb), hematocrit (HCT), platelet count

ABG includes: pH, PaCO2, PaO2, SaO2 , HCO3-

Liver profile includes: AST, ALT, ALP

Cardiac enzymes include: CPK, CK-MB, troponin I,

Lipid profile includes: total cholesterol, triglycerides, LDL, HDL

I’ve gone ahead and gathered the 2009 lab values published by the National Board of Medical Examiners and posted them below. Print them off and familiarize yourself with them as you’re going through your course work in your first and second years. You’ll be glad you did.

Yours for higher grades and an infinitely better lifestyle in medical school,

Cesar E. Orellana

Publisher and editor

www.MedicalSchoolReport.com

PS: For a helpful resource related to this article, visit: www.medicalschoolreport.com/free-book

USMLE Laboratory Values

* Included in the Biochemical Profile (SMA-12)

BLOOD, PLASMA, SERUM	REFERENCE RANGE	SI REFERENCE INTERVALS
* Alanine aminotransferase (ALT), serum	8–20 U/L	8–20 U/L
Amylase, serum	25–125 U/L	25–125 U/L
* Aspartate aminotransferase (AST), serum	8–20 U/L	8–20 U/L
Bilirubin, serum (adult) Total // Direct	0.1–1.0 mg/dL // 0.0–0.3 mg/dL	2–17 µmol/L // 0–5 µmol/L
* Calcium, serum (Ca²⁺)	8.4–10.2 mg/dL	2.1–2.8 mmol/L
* Cholesterol, serum	Rec:<200 mg/dL	<5.2 mmol/L
Cortisol, serum	0800 h: 5–23 µg/dL // 1600 h: 3–15 µg/dL	138–635 nmol/L // 82–413 nmol/L
	2000 h: < 50% of 0800 h	Fraction of 0800 h: < 0.50
Creatine kinase, serum	Male: 25–90 U/L	25–90 U/L
	Female: 10–70 U/L	10–70 U/L
* Creatinine, serum	0.6–1.2 mg/dL	53–106 µmol/L
Electrolytes, serum
Sodium (Na⁺)	136–145 mEq/L	136–145 mmol/L
* Potassium (K⁺)	3.5–5.0 mEq/L	3.5–5.0 mmol/L
Chloride (Cl^-)	95–105 mEq/L	95–105 mmol/L
Bicarbonate (HCO₃^-)	22–28 mEq/L	22–28 mmol/L
Magnesium (Mg²⁺)	1.5–2.0 mEq/L	0.75–1.0 mmol/L
Estriol, total, serum (in pregnancy)
24–28 wks // 32–36 wks	30–170 ng/mL // 60–280 ng/mL	104–590 nmol/L // 208–970 nmol/L
28–32 wks // 36–40 wks	40–220 ng/mL // 80–350 ng/mL	140–760 nmol/L // 280–1210 nmol/L
Ferritin, serum	Male: 15–200 ng/mL	15–200 µg/L
	Female: 12–150 ng/mL	12–150 µg/L
Follicle-stimulating hormone, serum/plasma	Male: 4–25 mIU/mL	4–25 U/L
	Female: premenopause 4–30 mIU/mL	4–30 U/L
	midcycle peak 10–90 mIU/mL	10–90 U/L
	postmenopause 40–250 mIU/mL	40–250 U/L
Gases, arterial blood (room air)
pH	7.35–7.45	[H⁺] 36–44 nmol/L
Pco₂	33–45 mm Hg	4.4–5.9 kPa
Po₂	75–105 mm Hg	10.0–14.0 kPa
* Glucose, serum	Fasting: 70–110 mg/dL	3.8–6.1 mmol/L
	2-h postprandial: < 120 mg/dL	< 6.6 mmol/L
Growth hormone - arginine stimulation	Fasting: < 5 ng/mL	< 5 µg/L
	provocative stimuli: > 7 ng/mL	> 7 µg/L
Immunoglobulins, serum
IgA	76-390 mg/dL	0.76–3.90 g/L
IgE	0–380 IU/mL	0–380 kIU/L
IgG	650–1500 mg/dL	6.5–15 g/L
IgM	40–345 mg/dL	0.4–3.45 g/L
Iron	50–170 µg/dL	9–30 µmol/L
Lactate dehydrogenase, serum	45–90 U/L	45–90 U/L
Luteinizing hormone, serum/plasma	Male: 6–23 mIU/mL	6–23 U/L
	Female: follicular phase 5–30 mIU/mL	5–30 U/L
	midcycle 75–150 mIU/mL	75–150 U/L
	postmenopause 30–200 mIU/mL	30–200 U/L
Osmolality, serum	275–295 mOsmol/kg H₂O	275–295 mOsmol/kg H₂O
Parathyroid hormone, serum, N-terminal	230–630 pg/mL	230–630 ng/L
* Phosphatase (alkaline), serum (p-NPP at 30°C)	20–70 U/L	20–70 U/L
* Phosphorus (inorganic), serum	3.0–4.5 mg/dL	1.0–1.5 mmol/L
Prolactin, serum (hPRL)	< 20 ng/mL	< 20 µg/L
* Proteins, serum
Total (recumbent)	6.0–7.8 g/dL	60–78 g/L
Albumin	3.5–5.5 g/dL	35–55 g/L
Globulin	2.3–3.5 g/dL	23–35 g/L
Thyroid-stimulating hormone, serum or plasma	0.5–5.0 µU/mL	0.5–5.0 mU/L
Thyroidal iodine (¹²³I) uptake	8%–30% of administered dose/24 h	0.08–0.30/24 h
Thyroxine (T₄), serum	5–12 µg/dL	64–155 nmol/L
Triglycerides, serum	35–160 mg/dL	0.4–1.81 mmol/L
Triiodothyronine (T₃), serum (RIA)	115–190 ng/dL	1.8–2.9 nmol/L
Triiodothyronine (T₃) resin uptake	25%–35%	0.25–0.35
* Urea nitrogen, serum	7–18 mg/dL	1.2–3.0 mmol/L
* Uric acid, serum	3.0–8.2 mg/dL	0.18–0.48 mmol/L

USMLE Laboratory Values

HEMATOLOGIC	REFERENCE RANGE	SI REFERENCE INTERVALS
Bleeding time (template)	2–7 minutes	2–7 minutes
Erythrocyte count	Male: 4.3–5.9 million/mm³	4.3–5.9 x 10¹²/L
	Female: 3.5–5.5 million/mm³	3.5–5.5 x 10¹²/L
Erythrocyte sedimentation rate (Westergren)	Male: 0–15 mm/h	0–15 mm/h
	Female: 0–20 mm/h	0–20 mm/h
Hematocrit	Male: 41%–53%	0.41–0.53
	Female: 36%–46%	0.36–0.46
Hemoglobin A₁_c	< 6%	< 0.06%
Hemoglobin, blood	Male: 13.5–17.5 g/dL	2.09–2.71 mmol/L
	Female: 12.0–16.0 g/dL	1.86–2.48 mmol/L
Hemoglobin, plasma	1–4 mg/dL	0.16–0.62 mmol/L
Leukocyte count and differential
Leukocyte count	4500–11,000/mm³	4.5–11.0 x 10⁹/L
Segmented neutrophils	54%–62%	0.54–0.62
Bands	3%–5%	0.03–0.05
Eosinophils	1%–3%	0.01–0.03
Basophils	0%–0.75%	0–0.0075
Lymphocytes	25%–33%	0.25–0.33
Monocytes	3%–7%	0.03–0.07
Mean corpuscular hemoglobin	25.4–34.6 pg/cell	0.39–0.54 fmol/cell
Mean corpuscular hemoglobin concentration	31%–36% Hb/cell	4.81–5.58 mmol Hb/L
Mean corpuscular volume	80–100 µm³	80–100 fL
Partial thromboplastin time (activated)	25–40 seconds	25–40 seconds
Platelet count	150,000–400,000/mm³	150–400 x 10⁹/L
Prothrombin time	11–15 seconds	11–15 seconds
Reticulocyte count	0.5%–1.5%	0.005–0.015
Thrombin time	<2 seconds deviation from control	<2 seconds deviation from control
Volume
Plasma	Male: 25–43 mL/kg	0.025–0.043 L/kg
	Female: 28–45 mL/kg	0.028–0.045 L/kg
Red cell	Male: 20–36 mL/kg	0.020–0.036 L/kg
	Female: 19–31 mL/kg	0.019–0.031 L/kg

USMLE Laboratory Values

CEREBROSPINAL FLUID	REFERENCE RANGE	SI REFERENCE INTERVALS
Cell count	0–5/mm³	0–5 x 10⁶/L
Chloride	118–132 mEq/L	118–132 mmol/L
Gamma globulin	3%–12% total proteins	0.03–0.12
Glucose	40–70 mg/dL	2.2–3.9 mmol/L
Pressure	70–180 mm H₂O	70–180 mm H₂O
Proteins, total	<40 mg/dL	<0.40 g/L

USMLE Laboratory Values

SWEAT	REFERENCE RANGE	SI REFERENCE INTERVALS
Chloride	0–35 mmol/L	0–35 mmol/L

URINE
Calcium	100–300 mg/24 h	2.5–7.5 mmol/24 h
Chloride	Varies with intake	Varies with intake
Creatinine clearance	Male: 97–137 mL/min
	Female: 88–128 mL/min
Estriol, total (in pregnancy)
30 wks	6–18 mg/24 h	21–62 µmol/24 h
35 wks	9–28 mg/24 h	31–97 µmol/24 h
40 wks	13–42 mg/24 h	45–146 µmol/24 h
17-Hydroxycorticosteroids	Male: 3.0–10.0 mg/24 h	8.2–27.6 µmol/24 h
	Female: 2.0–8.0 mg/24 h	5.5–22.0 µmol/24 h
17-Ketosteroids, total	Male: 8–20 mg/24 h	28–70 µmol/24 h
	Female: 6–15 mg/24 h	21–52 µmol/24 h
Osmolality	50–1400 mOsmol/kg H₂O
Oxalate	8–40 µg/mL	90–445 µmol/L
Potassium	Varies with diet	Varies with diet
Proteins, total	<150 mg/24 h	<0.15 g/24 h
Sodium	Varies with diet	Varies with diet
Uric acid	Varies with diet	Varies with diet

BODY MASS INDEX (BMI)	Adult: 19–25 kg/m²