The presentation of an amoebic liver abscess, as we have already seen, is so diverse that it can be encountered by a specialist in any branch of medicine or surgery. Depending on the site of an abscess its differential diagnosis will vary. Thus, an adequate discussion on this subject could cover half the field of internal medicine.
The conditions that may mimic a superior surface abscess of the right or the left lobe are quite varied. Similarly, the differential diagnosis of an abscess in the inferior surface of the right or the left lobe is quite different. Again, if the patient presents with a complication of an amoebic abscess like pericarditis, pleurisy, pneumonia or acute abdomen, then one's attention is focussed on an entirely different set of conditions. To avoid confusion, an attempt has been made to classify the long list in a practical manner. Many of the conditions have already been discussed earlier and some repetition is unavoidable.
The conditions most likely to be confused with an amoebic liver abscess are pyogenic liver abscess, subphrenic abscess, malignancy of the liver, or acute cholecystitis.

Pyogenic liver abscess
It may be impossible or difficult to differentiate these two illnesses especially in countries where amoebic abscess is uncommon. In areas where amoebiasis is endemic, however, one should favour the diagnosis of an amoebic abscess even if the aspirated pus shows the presence of bacteria, as these could be present if the abscess was secondarily infected .
Usually pyogenic abscess gives rise to high spiky fever, chills, prostration, pain and tenderness in the liver area, and jaundice. The onset of the illness is more acute and the patient is very toxic. Often the illness is associated with intra-abdominal sepsis, appendicitis, cholecystitis, etc.
A marked polymorphonuclear leucocytosis, positive blood culture, elevated serum enzymes and alkaline phosphatase are characteristic of this condition.

Right sided subphrenic abscess
It is a difficult task to exclude this condition. With its features of pain and tenderness in the liver area, right shoulder pain, associated chest signs, it very closely mimics an amoebic abscess of the superior surface of the right lobe of the liver. Even a radiograph showing a raised hazy immobile diaphragm, will not help in the differential diagnosis.
A preceding history of any abdominal operation, inflammatory or perforating conditions of intra-abdominal viscera or abdominal trauma may suggest the diagnosis of a subphrenic abscess. A lung and liver scan may help in differentiating it from an amoebic liver abscess.

Malignancy of the liver
This may either be a primary, i.e. hepatoma, or secondary metastatic deposits in the liver.
A hard, nodular and markedly enlarged liver with or without jaundice should suggest malignancy.
Patients with an acute onset of symptoms and signs mimicking an amoebic liver abscess have been reported from some parts of Africa. There the incidence of hepatoma appears to be rising very rapidly. In a majority of patients presenting with symptoms of short duration, the consideration of hepatoma does not arise. However, in a small percentage of cases of amoebic liver abscess with a long standing history and an irregular, hard, tender enlarged liver, it may become impossible to differentiate the two conditions. In one of my patients diagnosed and treated as amoebic liver abscess, 30 ml chocolate coloured pus mixed with serum was aspirated (Fig. 1). Liver biopsy was done due to unusual presence of serum with pus. The diagnosis turned out to be undifferentiated malignancy of the liver. If facilities are available, ^99m Tc colloid liver scan followed by ^113mln scan or dynamic blood flow studies should be done.
I.H.A. test and presence of alpha-feto proteins in blood should also be determined. In centres where nuclear medicine studies are not available, selective coeliac arteriography would help to study the vascularity of the mass in the liver, hepatomas being very vascular. In others, liver biopsy decides the issue.

Acute cholecystitis
A sudden onset of severe pain in right upper quadrant, nausea, vomiting, fever and minimal icterus should suggest the diagnosis of acute cholecystitis. In non-endemic areas, with a past history of similar disturbances, the diagnosis becomes almost certain, and amoebic liver abscess will be highly unlikely.

Differential diagnosis of amoebic liver abscess with a lump in the liver

Hydatid cyst of the liver
Patients with a hydatid cyst usually present with a lump in the upper abdomen. Fever, pain and tenderness-the cardinal features of an amoebic liver abscess, are often absent. Theoretically the differential diagnosis between the two conditions appears simple and on many occasions it is so. However, we often see patients who interpret the dull, dragging discomfort of hydatid cyst as pain and wince, when the lump is palpated to elicit tenderness. Figure 2 shows a hydatid cyst removed from a patient, who had a tender lump in the epigastrium. Figure 2a shows a very dense cold area in the ¹³¹l Rose Bengal liver scan. In my opinion an amoebic liver abscess does not produce such a dense cold area with this isotope. Thus, in this case, it was considered more likely to be a cyst. In spite of the fact that a Casoni's test was negative (not uncommon in patients with hydatid cyst), no needling was done. Instead a laparotomy was decided upon.
Non-parasitic cysts of the liver should be differentiated on the same lines.

Differential diagnosis of amoebic liver abscess with a tender smooth hepatomegaly
Alcoholic hepatitis, viral hepatitis and a congested liver of congestive cardiac failure or constrictive pericarditis are most likely to be mistaken for amoebic liver abscess in patients having a smooth, tender and enlarged liver.

Alcoholic hepatitis
This condition is relatively uncommon, occurring in heavy drinkers. However, as patients of amoebic liver abscess frequently give history of consuming alcohol, at times it becomes very difficult to exclude alcoholic hepatitis. Patients may complain of fever, pain in the hepatic area and all other symptoms of an amoebic liver abscess. Although one finds a tender hepatomegaly in both the conditions, jaundice is more frequent in alcoholic hepatitis. Liver scan, if available, would immediately exclude an amoebic liver abscess. There is no specific treatment for alcoholic hepatitis and, therefore, in cases where the diagnosis is not clear, it is advisable to start anti-amoebic therapy.

Viral hepatitis
Although amoebic liver abscess and viral hepatitis are both common in under-developed countries, differentiating the two is not difficult. Complete loss of appetite, nausea, vomiting and absence of pain, all favour viral hepatitis. At this early stage, serum enzymes like S.G.P.T. are markedly elevated. Once jaundice appears, the differentiation becomes further simplified.

Congested liver of congestive cardiac failure or constrictive pericarditis
This may, at times, present as an amoebic liver abscess. Most of these patients have a diffuse enlargement of both the lobes. This is less common in amoebic liver abscess. Absence of fever and severe pain and presence of engorged neck veins along with other signs in the chest suggesting involvement of the heart, will aid in the diagnosis. On three occasions,such problem patients were referred to me in the nuclear medicine department for a liver scan to exclude amoebic liver abscess. These were known cases of chronic advanced congestive cardiac failure in whom the clinician palpated a tender lump on the anterior surface of a markedly enlarged and tender liver. The scan did not show any cold area.
Figure 3 shows a liver at autopsy in one of these cases. The increased convexity on the anterior surface was mistaken for the lump.

Differential diagnosis of inferior surface left lobe abscess

1. In our experience, a hepatoma of the left lobe is the nearest differential diagnosis and is quite often difficult to exclude. A liver scan does not help very much in differentiating these conditions. At times, we have confidently introduced a needle expecting to strike pus and instead found bloody fluid commonly obtained from hepatomas. (Fig. 19 section IV). Peritoneoscopy has often helped us to exclude an abscess. Biopsy taken through a peritoneoscope. clinches the diagnosis.
2. Hydatid cyst has already been discussed.
3. Figure 4 shows the scan of a female patient with diffuse epigastric lump diagnosed as an amoebic liver abscess. Ultimately at laparotomy a pseudopancreatic cyst (Fig. 5) was detected.
4. Figure 11 shows another abdominal lump diagnosed as a junctional inferior surface amoebic liver abscess. The liver scan showed a junctional cold area (Fig. 12). Finally, it proved to be a pyogenic abscess in the rectus sheath.

Differential diagnosis of an amoebic liver abscess of the inferior surface of the right lobe
A case diagnosed as an inferior surface amoebic liver abscess of the right lobe showed a cold area on the liver scan (Fig. 13) which was not very convincing. Hence other investigations were done. Barium enema showed (Fig. 14) a lesion in the ascending colon and hepatic flexure compatible with a colonic mass. At laparotomy this turned out to be a tuberculous lesion with paracolic abscess. Differential diagnosis of a superior surface right lobe amoebic liver abscess
Figures 15 a,b,c,d show the chest X-rays of patients having abnormal shadows at the right base, referred to us for liver scan to exclude an amoebic liver abscess. The scans were normal in these patients Clinically, it was very difficult to rule out amoebic liver abscess.
Liver scanning thus plays a very important role in patients having such shadows in the chest. If this investigation is not available, then the examination of the pleural fluid and/or the sputum for presence of E. Histolytica should be done. At the present stage of our experience, we cannot rule out the possibility of missing a small superficial amoebic liver abscess in the superior surface on liver scan. In such cases, Ultrasonic and CAT scans also may not be very helpful.
Figures 16 a,b,c are the chest X-rays of patients with vague symptoms in whom abnormal bulges in the right dome of the diaphragm suggested a probable amoebic liver abscess. In all these cases, liver scan did not show a cold area, thus ruling out an amoebic liver abscess. These patients had a partial or a complete eventration of the diaphragm.
Patients of amoebic liver abscess with acute onset of high fever, pain in the right lower chest and dry cough, can present as a close differential diagnosis of pneumonia. Tachypnoea, alae nasi sign and absence of an elevated right dome would be in favour of the latter.

Differential diagnosis of an amoebic liver abscess in the postero-inferior surface of the right lobe

1. A cold abscess is less likely if acute pain and marked tenderness are present.
2. A perinephric abscess has to be considered in the differential diagnosis. Symptoms referable to urinary tract, if present, spasm of the psoas muscle, and history of diabetes, should make one suspicious of a perinephric abscess. Plain X-ray abdomen and l.V.P. would clinch the diagnosis.

Differential diagnosis of patients presenting with general symptoms

1. (a) In patients with an acute onset of fever, illnesses such as flu, malaria, typhoid, etc. would have to be excluded.
   (b) In patients with a gradual onset of fever over a few weeks, tuberculosis must be eliminated first. All the other causes of P.U.O. would also need to be excluded.
2. In patients with an "acute abdomen"-all other causes of this condition would have to be ruled out-the common ones being perforation of a peptic ulcer or an appendix.
3. In patients with a lump in the upper abdomen -the differential diagnosis would include all tuberculous or malignant lumps in this region. Splenic abscess though a rare entity closely simulates a left lobe abscess and has been discussed earlier.
4. In patients with empyema of the right side of the chest, a complicated superior surface amoebic liver abscess of the right lobe has always to be excIuded .
5. In patients of amoebic liver abscess presenting with obstructive jaundice, the usual causes of this condition would have to be excluded-the common ones being gall stones or pancreatic swellings; and the most uncommon being a choledo chus cyst. Figure 17 shows a rare choledochus cyst arising from the right lobe of the liver seen at operation. This 53 years old male patient was diagnosed as having an amoebic liver abscess. Tapping showed fluid similar to bile, which we have seen in the past in cases of an amoebic liver abscess (Sect. IV. Figs. 16 a,b). Since the lump in the liver filled up again, we excluded an amoebic liver abscess and thought of a choledochus cyst.
6. In patients with hepatic or hepato-renal failure, common causes like viral hepatitis or cirrhosis of liver will have to be eliminated.