Introduction: Why this guide?

Five years ago, new evidence about vitamin D alerted doctors to the impacts of vitamin D deficits on patient health. This drove a meteoric increase in vitamin D testing in labs across North America. At some clinics, vitamin D testing became one of the single biggest expenses in the budget for lab services.

For all those tests and all that money, you would expect that doctors were at least getting useful information from the results. But they weren’t.

Vitamin D levels are not good predictors of bone health—or other health outcomes for that matter. Patients have different underlying disease susceptibilities, and, depending on supplements, their serum vitamin D levels go up and down. So, knowing a patient’s current vitamin D level has dubious value.

Ample evidence, however, shows the benefits of vitamin D supplements for almost everyone, regardless of their baseline vitamin D level. Therefore, a strategy of “treat don’t test” makes eminent sense and saves valuable health-care resources for testing that actually has a clinical impact.

In the setting of finite health-care resources, tradeoffs such as this—between clinical utility and costs of testing—will become increasingly important.

As health practitioners, we share an ethical responsibility to provide good stewardship of limited health-care dollars and testing resources. Whether you are a primary-care physician or resident, a medical student, or a health professional in an allied field, you need to be lab literate: you need to know which tests have the highest yield for the clinical situations you typically encounter.

Many references help you interpret lab investigations, but they don’t tell you what investigations to do in the first place. This guide is about what lab investigations to do first. We outline the most efficient and cost-effective way for you to use laboratory investigations to support clinical diagnosis and management.

How to use this guide The main guide

The guide is organized the way clinicians think: by clinical presentation and by organ system. So, if you have a patient with a skin problem, go to the section on dermatology. If a patient presents with fatigue, go to the section on fatigue.

In addition to information on lab investigations, we provide, where useful, differential diagnoses, etiologies, and summaries of signs and symptoms. We also share some “pearls”—particular knowledge about lab investigations we have gathered as experts and clinicians in our fields.

Lab basics

Lab investigations are only as good as the specimens delivered for analysis, and lab results are only as useful as human slip-ups and margins of error allow.

Find advice and information here on lab errors, false positives and negatives, and blood and tissue collection.

Lab investigations index

This index describes the diagnostic purpose of the lab investigations discussed in the guide, plus other common lab tests.

If you need a quick check on what an investigation is for, look it up here.

What’s not in this guide

This guide focuses on laboratory investigations. It does not cover diagnostic imaging.

It covers typical disorders and clinical presentations. It does not cover every disorder and clinical presentation, and is not meant to replace sound clinical judgement.

A note about units

This guide gives laboratory values in both conventional units and SI units (the International System of Units). We give the conventional units first and the SI units second.

We did this to be thorough: US laboratories usually report test results in conventional units, but SI units are increasingly used in the United States.

For this reason, we felt it was important to provide laboratory values in both systems as a reference.