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135 to 145 mEq/L. Sodium mostly reflects water balance in the body. Low sodium (hyponatremia) can cause headache, confusion and, if severe, seizures. High sodium (hypernatremia) goes with thirst, dry mucous membranes and confusion, often from dehydration. Watch neuro status, fluid intake and output, and daily weight.

3.5 to 5.0 mEq/L. The range is narrow and both ends affect the heart. Low potassium causes weakness, cramps, constipation and arrhythmias, with flat T waves and U waves on the ECG. High potassium causes weakness and arrhythmias, with peaked T waves and a wide QRS. Watch heart rhythm. A hemolyzed blood sample can read falsely high.

8.5 to 10.5 mg/dL. Low calcium: numbness and tingling around the mouth, muscle cramps, tetany, and positive Chvostek and Trousseau signs. High calcium: bones, stones, groans and moans, meaning bone pain, kidney stones, constipation and abdominal pain, and confusion. Low albumin lowers total calcium, and ionized calcium is the active form.

1.5 to 2.5 mg/dL. Low magnesium often travels with low potassium and low calcium, and brings tremor, twitching and arrhythmias. High magnesium is usually seen with kidney impairment, because the kidneys clear it, and brings loss of deep tendon reflexes, drowsiness, low blood pressure and slowed breathing. Check reflexes and respiratory rate.

2.5 to 4.5 mg/dL. Phosphorus and calcium have an inverse, seesaw relationship: when one goes up the other tends to go down. High phosphorus is most often seen with kidney disease. Low phosphorus causes muscle weakness and can happen when a malnourished person restarts eating (refeeding).

Hemoglobin: about 13.5 to 17.5 g/dL in men and 12 to 16 g/dL in women. Hematocrit: about 41 to 53 percent in men and 36 to 46 percent in women. The rule of three says hematocrit is roughly three times hemoglobin. Low values mean anemia: fatigue, pallor, shortness of breath, fast heart rate. High values suggest dehydration or too many red cells. After sudden bleeding the values can look normal at first.

About 4,500 to 11,000 per mm3. A high count (leukocytosis) points to infection, inflammation or stress, and some medications such as steroids raise it. A low count (leukopenia) raises infection risk. For neutrophils specifically, an absolute neutrophil count under 1,500 is neutropenia and under 500 is severe. Watch temperature and infection signs.

About 150,000 to 400,000 per mm3 (some labs go up to 450,000). Low platelets (thrombocytopenia) raise bleeding risk: petechiae, easy bruising, bleeding gums and nosebleeds. The risk climbs as the count falls, and counts below about 20,000 carry a risk of spontaneous bleeding. Watch for bleeding and protect the skin and mucous membranes.

PT is about 11 to 13.5 seconds. INR is about 0.8 to 1.1 in people who are not on anticoagulants. PT and INR are used to monitor warfarin, and the target INR is set by the prescriber for each patient (many common targets are 2.0 to 3.0). A high INR means blood clots more slowly: watch for bruising and blood in urine or stool. Liver disease and low vitamin K also raise PT.

About 25 to 35 seconds, though labs differ and some run 30 to 40. aPTT is used to monitor heparin, with a target often expressed as 1.5 to 2.5 times the control value, set by protocol and the prescriber. A prolonged aPTT means a bleeding risk. A quick way to remember it: warfarin goes with PT and INR, heparin goes with aPTT.

10 to 20 mg/dL. High BUN suggests dehydration, reduced kidney function, GI bleeding or a high-protein intake. Low BUN is seen with liver disease, malnutrition and overhydration. The BUN-to-creatinine ratio is normally about 10:1 to 20:1, and a ratio above that points toward dehydration or poor blood flow to the kidneys.

About 0.6 to 1.2 mg/dL, lower in women and higher in men because it tracks muscle mass. The body makes creatinine at a steady rate and the kidneys clear it, so it is less affected by diet and hydration than BUN. A rising creatinine means the kidneys are clearing less. As a rough rule, a doubled creatinine means about half the kidney function.

An eGFR of 90 or more mL/min/1.73 m2 is normal. An eGFR below 60 for three months or longer points to chronic kidney disease, and below 15 is kidney failure (stage 5). eGFR is calculated from creatinine, age and sex, so it estimates how well the kidneys filter blood.

7.35 to 7.45. Below 7.35 is acidosis, and above 7.45 is alkalosis. The pH tells you which way the blood is leaning. Then check PaCO2 and bicarbonate to see whether the lungs (respiratory) or the kidneys and metabolism (metabolic) explain it.

35 to 45 mmHg. This is the respiratory part, and CO2 acts as an acid. A high PaCO2 means respiratory acidosis, from breathing too little (hypoventilation), as with sedation or COPD. A low PaCO2 means respiratory alkalosis, from breathing too fast, as with pain, anxiety or fever. Watch breathing rate and effort.

22 to 26 mEq/L. This is the metabolic part, and bicarbonate acts as a base. A low HCO3 means metabolic acidosis, as in diabetic ketoacidosis, severe diarrhea or kidney failure. A high HCO3 means metabolic alkalosis, as with prolonged vomiting or nasogastric suction.

PaO2 is 80 to 100 mmHg and SaO2 is 95 to 100 percent. Low values mean hypoxemia: restlessness, confusion, fast breathing and a fast heart rate. A pulse oximeter reading (SpO2) estimates SaO2 without a blood draw. Some patients with chronic lung disease live with lower baselines, so compare with their own normal.

First look at pH: acidosis or alkalosis. Then ROME: in Respiratory problems the pH and CO2 move in Opposite directions (low pH with high CO2 is respiratory acidosis). In Metabolic problems the pH and HCO3 move the Equal way (low pH with low HCO3 is metabolic acidosis). Whichever one matches the pH is the cause.

Uncompensated: pH is abnormal and only one of CO2 or HCO3 is off. Partially compensated: pH is still abnormal, but both CO2 and HCO3 are off, because the other system has started to respond. Fully compensated: pH is back in the normal range, but CO2 and HCO3 are both still abnormal. The lungs respond within minutes to hours, the kidneys over days.

Fasting glucose is 70 to 99 mg/dL. Below 70 is hypoglycemia: shakiness, sweating, hunger, confusion and a fast heartbeat. By American Diabetes Association criteria, a fasting level of 126 or higher on two tests points to diabetes, and 100 to 125 is prediabetes. A1c reflects the average over about 3 months: below 5.7 percent is normal, 5.7 to 6.4 is prediabetes, and 6.5 or higher is diabetes.